The remaining transmitting apparatus consists of the battery D^2 and the

usual wireless apparatus. The double-pole two-way switch B' is to enable

the photo-telegraphic set to be switched out and the hand key W switched in

for ordinary signalling purposes. The battery D^2 should be about 12 volts.

RECEIVING APPARATUS

The wireless portion of the receiver is similar to that given in Fig. 22,

is of the usual syntonic type, and comprises an oscillation transformer, S

being the secondary, and P the primary; C' is a block condenser, and C a

variable condenser. The detector D is of the carborundum crystal or

electrolytic pattern. A two-way switch B is provided so that the relay U

can be switched out and the telephones J switched in for ordinary receiving

purposes. The relay U is a Brown's telephone relay.

[Illustration: FIG. 40.]

_The Receiver._--The magnified current from the relay U is taken to a

special telephone receiver, the construction of which is given in Fig. 40.

The diaphragm F is about 2-1/2 inches diameter, and should be fairly thin

but very resilient. Only one {84} [Illustration] [Illustration] coil is

provided, and this should be wound with No. 47 S.S.C. copper wire for a

resistance of about 2000 ohms. By using only one coil and therefore only

one core, the movement of the diaphragm is centralised. To the centre of

the diaphragm a light steel point is fastened, about 1/2 inch long, and

provided with a projecting hook H. An enlarged view of this pin is given in

Fig. 41. The movement of the diaphragm and consequently of the steel point

P is communicated to a pivoted rod R, which is of special construction. A

piece of aluminium tube 3-3/4 inches long, and of the section given at B,

is bushed at one end with a piece of brass of the shape shown in Fig. 41a.

A stiff steel wire T about 1 inch long (20 gauge) is screwed into the end

of Z, and carries a counterbalance weight C. A hardened {85} steel spindle,

pointed at both ends, is fastened at D, and runs between two coned

bearings, one of which is adjustable. The underside of Z is flattened, and

a small coned depression is made for the reception of the pointed end of

the pin. By means of the spring J the two pieces, Z and P, are held firmly

together, at the same time allowing perfect freedom of movement. The bridge

G is made from a piece of sheet aluminium placed in a slot cut in the tube

R, the end of the tube being pressed tight upon G, and secured by means of

a small rivet.

The optical arrangements are as follows. By means of the Nernst lamp L, and

the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown

upon the screen J. When the shutter G is in its normal position (_i.e._ at

rest), its shadow is just above the small hole in J, and light from L

reaches the photographic film wrapped round the drum V of the machine.

[Illustration: FIG. 42.

J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B_1, focussing

lens.]

When, however, signals are sent out from the transmitting apparatus, the

magnified current from the relay U energises the coil of the special

telephone S, attracting the diaphragm F, and consequently giving movement

to the pivoted rod R. As by means of the optical arrangements a {86}

magnified movement as well as a magnified image of G is thrown upon the

screen J, the shadow of G will, when the telephone S is actuated, cover the

hole in the screen, and prevent any light from reaching the film on V,

until current from the relay U ceases to flow. Therefore, when the stylus

of the transmitter traces over a conducting strip on the metal print, no

light reaches the film on V, but when tracing over an insulating strip the

shadow of G on the screen J rises, and the light from L reaches the film.

By this means a positive picture is received, which is a great advantage

where the photographs are required for reproduction. Atmospherics would be

represented by irregular transparent marks on the film after development,

and these can be easily eradicated by retouching.

[Illustration: FIG. 43. E, ebonite screen; F, focussing lens; G, shutter;

O, condensing lens; L, Nernst lamp.]

The drum of the machine moves laterally 1/75th of an inch per revolution,

and the hole in the screen is 1/90th of an inch in diameter. As the screen

J is not in direct contact with the film, the slight diffusion of the light

that takes place will produce {87} a mark of about the right thickness.

With a movement of the diaphragm of only 1/40000th of an inch, the actual

movement of G will be 1/4000th of an inch. If the optical arrangements have

a magnifying power of 100, then the movement of the shadow upon the screen

will be 1/40th of an inch, which will be ample to cover the aperture.

The aluminium rod R, minus the counter-weight, can be made to weigh not

more than 12 grains. It is necessary to enclose the optical parts in a

light tight box, indicated by the dotted lines in Fig. 43, in order to

prevent any extraneous light from reaching the film.

_The Contact Breaker._--The contact breaker (L, Fig. 35), as will be seen

from Fig. 44, consists of an electro-magnet N, the windings of which are

connected with the battery B and the polarised relay K. The armature which

is supported by the spring G carries a contact arm A, which in its normal

position makes permanent contact with the contact screw T, and completes

the circuit between the relay K and the telephone relay U (Fig. 35). As

soon as the transmitter sends out the first signal, the magnified current

from the telephone relay actuates the relay K, which in turn completes the

circuit of the contact breaker. Directly the armature M has been attracted,

the contact with T is broken, and A makes fresh contact with the screw H,

by means of the spring Z {88} fastened to the underside of A. The armature,

once it has been attracted, is held in permanent contact with H by the

catch S, independent of the magnets N. As soon as contact is made with H,

the clutch (F, Fig. 35) circuit is completed, and the circuit of the relay

K is broken. When the circuit of the clutch F is broken by means of the

circuit breaker C on the machine (Fig. 36), the stop S is pulled back by

hand, allowing the contact arm A to rise, and again make fresh contact with

the contact screw T.

[Illustration: FIG. 44.]

DRIVING APPARATUS

_The Friction Brake._--This consists of a steel disc A, Fig. 45, about

2-1/2 inches diameter and 3/8 inch or 1/2 inch wide on the face, secured to

the main shaft of the driving motor. The arm H, pivoted at C, carries at

one end the curved block B, which is faced with a pad of tow F. The other

extremity is pivoted to the steel rod P, which slides {89} [Illustration]

in holes bored in the standards J. One end of the rod P is screwed with a

fine thread, about 75 to the inch, and is fitted with a regulating wheel T,

by means of which the block B can be made to press upon the disc A with any

required degree of pressure. A fairly stiff steel spring R is placed upon

the rod P, between one standard J and the collar N. As the speed of the

driving motor is slightly in excess of that required by the machine, the

block B, by means of the wheel, is made to press upon the disc A, setting

up friction which reduces the motor speed until the isochroniser indicates

that the correct working speed has been attained.

_The Clutch_.--The details of this will be seen from Figs. 46 and 47. It

consists of a steel shaft coned at both ends running between two

countersunk bearings, one of which is adjustable. This shaft carries the

two portions of the clutch A and B, the portion A being a fixture on the

shaft, and the portion B running free upon it. The portion B is a gun-metal

casting bored to run accurately upon the steel shaft. A soft iron annular

ring is fastened to the face.

[Illustration: FIG. 46.

E, spindle; R, bobbins; P, iron cores; D, copper rings; T, brushes; N, back

plate; V, front plate; J, gearing; S, spring; H, collar; Z, iron ring; F,

fixed bearing; C, insulating bush.]

The portion A consists of a gun-metal casting {90} [Illustration] bored a

tight fit for the shaft E, secured by means of a set screw. The two magnet

cores P are screwed into the front plate V, which is also of gun-metal, and

after the bobbins R have been slipped on, the shanks of the cores are

passed through holes drilled in the flange N of the main casting and held

in place with nuts. The faces of both A and B must be turned perfectly

square with the shaft, so that they run accurately together. The portion B

is {91} kept in contact with A by means of a spring S, the pressure being

regulated by the collar H. Current is taken to the magnets by means of the

two insulated copper rings D mounted upon the body of A. The gear-wheels on

both portions have teeth of very fine pitch, the number of teeth on each

being regulated by the speed of the driving motor and the required machine

speed. Connection with the circuit breaker L and the battery B^2 is made

with the collecting rings D by the brushes T. The complete connections are

given in the diagram Fig. 51.

_The Isochroniser._--This is a device for ensuring the correct speed

regulation of the driving motors, and is shown in detail in Fig. 48. It

comprises two portions, one portion being rotated at a definite speed by

electrical means, and the other portion rotated by the driving motor.

The main portion consists of a metal tube N, bushed at both ends, the

bottom end of the tube being arranged to work on ball-bearings. An ebonite

bush C carries three copper rings T, T^1, T^2, and the brushes R, R^1, R^2

are in electrical contact with them. The ebonite plate J, 3-1/2 inches

diameter, is secured to the top end of N, and carries a contact piece Q,

shown separate at E. As will be seen this is a block of ebonite with three

contacts arranged on the top surface. The middle contact P is 1/64th of an

inch wide, and the contacts P^1 {92} and P^2 are placed on either side at a

distance of 1/16 inch; the contact strips P^1, P^2 carry the brass pins D,

which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting

wire is carried from the contact P to the copper ring T, another from P^1

to T^1, and one from P^2 to T^2.

[Illustration: FIG. 48.

N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T^1, T^2,

copper rings; C, insulating block; R, R^1, R^2, brushes; J, ebonite disc;

Q, contact block; D, metal pins; O, pulley, P, P^1, P^2, contact plates; K,

needle; Z, spring; W, steel rod; E, countersunk bearing.]

The bushes S are bored a running fit for the steel rod W (shown separate at

A), which is coned at both ends, and runs between two countersunk bearings,

the bottom bearing E being fixed while {93} the top bearing (not shown) is

adjustable. A needle K is fastened near the end of the rod W, and attached

to this needle is the spring Z, which presses lightly but firmly upon the

contact block Q. To provide a level surface for Z to work over, the spaces

between the contact pieces are filled in with an insulating material, and

the whole surface finished off perfectly smooth. The spring Z is 1/8 inch

wide for portion of its length, but at the point where it presses upon Q it

is reduced in width to 1/64th of an inch (see Fig. 48). The driving

arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a

grooved pulley, is run in bearings parallel with the shaft W, and is

connected by suitable gearing to the shaft of the driving motor, so that

the needle K makes one revolution in about 2-1/2 seconds. A belt passing

over the pulleys connects the two shafts, and the tension of the belt is

regulated by means of an adjustable jockey pulley.

The tube N, carrying the disc J, must be rotated at a fixed speed, and this

is accomplished in the following manner. An ordinary electric clock impulse

dial, actuated from a master clock, is connected by suitable gearing H, so

that the tube N makes exactly one revolution in 2 seconds; it being

possible to adjust an electric clock of the "Synchronome" type, so that it

only gains or loses about 1 second in 24 hours, and this provides {94} an

accuracy sufficient for all practical purposes. The connections are given

in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that

a connecting wire is run from the steel spindle W to one terminal each of

the lamps L, L^1, L^2, and from the other terminal of the lamps to one

terminal of the batteries J, the battery comprising a set of three 4-volt

accumulators. The other terminals of the batteries are joined one to each

of the brushes R, R^1, R^2.

[Illustration: FIG. 49.]

[Illustration: FIG. 50.

M, terminals for connecting to electric clock; L, white lamp; L^1, blue

lamp; L^2, red lamp.]

The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2

blue and red respectively, and care must be taken in connecting up that

when the needle K makes contact with the stud P the white lamp L is in

circuit. When the machines are working, the operator, by means of the brake

(already described), reduces the speed of the driving motor until the

needle K travels in unison with the disc J, making permanent contact with P

on the contact {95} block Q, which is evidenced by the lamp L remaining

alight. If, however, the needle travels faster than the disc J, contact

with P is broken and fresh contact is made with P^2, the lamp L is

extinguished and the red lamp L^2 lights up, and remains alight until the

operator reduces the speed. Similarly, too, if the needle travels slower

than J, contact is made with P^1, and the circuit of the blue lamp L^1 is

completed. When the speed is either above or below the normal, the needle K

engages with one or the other of the pins D, and as the tension of the

driving belt is only such as is required to drive the needle, the belt

slips on the pulleys until the normal speed is regained.

METHOD OF WORKING

The clockwork motor M, Fig. 51, should be capable of running for several

hours with one winding, and powerful enough to take up the work of driving

the machine without any appreciable effort. The main spindle of the motor

is so arranged that it makes one revolution in two minutes, and the

reduction in speed between the motor shaft and the shaft to which the

coupling A is attached is 30:1. The metal line print having been wrapped

round the drum of the machine, the stylus is put into position, at the edge

of the lap, and with the needle resting about half-way on {96} the margin

of the bare foil left at the commencing edge of the print. Now, when the

two stations are in perfect readiness for work, the motors are started and

the speed adjusted; the speed of the machine being just under one

revolution in four seconds.

[Illustration: FIG. 51.

M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F,

electric clutch; X, gearing; D, D^1, switches; A, flexible coupling; K,

polarised relay; L, circuit breaker; B_1, B_2, B_3, batteries; P, electric

clock; W, terminals for connection to telephone relay; H, terminals for

connection to terminals J, on transmitting machine.]

The switch D is then closed, and the arm of the switch D^1 placed on the

contact stud (1), at the transmitting station only. As soon as the switches

are closed the clutch F comes into action, and the transmitting machine

begins to revolve. When the whole of the line print wrapped round the drum

of the machine has passed under the stylus, the end of the shaft D, Fig.

36, engages {97} with the spring _m_, breaking the clutch circuit and

allowing the motor to run free. As soon as the machine stops, the switch D

is opened and the machine run back to its starting position by hand.

At the receiving station the switch D is also closed, and the arm of the

switch D^1 placed on the contact stud (2). The closing of these switches

does not bring the clutch F into operation until current from the telephone

relay U connected to the wireless receiving apparatus works the sensitive

polarised relay K, which in turn completes the circuit of the

circuit-breaker L. When the armature of L is attracted, the circuit of the

relay K is broken, the circuit of the clutch F is completed, and the

machine starts revolving.

[Illustration: FIG. 52.]

The current from the relay U, due to the transmitting stylus passing over

_one_ contact strip on the metal print, is too brief to actuate the heavier

mechanism of the relay K, hence the need of the margin of bare foil at the

commencing edge of the metal print, so that a practically continuous

current will flow to the relay K until the armature is attracted. As,

however, the relay is not actuated at the receipt of the first signal, and

as it is necessary for the machine to start recording at a certain point on

the film, viz. {98} at the edge of the lap--the reason for this was given

in Chapter IV.--the starting position of the receiving drum will be similar

to that given in the diagram Fig. 52, where X indicates the lap of the

photographic film, and the arrow the direction of rotation.

It is, of course, obvious that a somewhat similar adjustment must be made

with regard to the position of the stylus on the metal print at the

transmitting machine.

In the present system, as in almost every photographic method of receiving

that has been described, the Nernst lamp is invariably mentioned as the

source of illumination. Since the advent of the high-voltage metal-filament

lamps the Nernst lamp has fallen somewhat into disuse for commercial

purposes, but it possesses certain characteristics that render it eminently

suitable for the purpose under discussion.

The main principle of this type of lamp depends upon the discovery made by

Professor Nernst in 1898, after whom the lamp is named, that filaments of

certain earthy bodies when raised to a red heat became conductive

sufficiently well to pass a current which raised it to a white heat, and

furthermore that the glowing filament emitted a brighter light for a given

amount of current than carbon filaments.

[Illustration: FIG. 52a.]

Nernst lamps are made in two sizes, the larger {99} being intended for the

same work as usually done by arc lamps, and the smaller to replace

incandescent lamps; the smaller type being made to fit into the ordinary

bayonet lampholders. The principal parts of a Nernst lamp consist of the

filament, the heater, the automatic cut-out, and the resistance, and their

arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.

The current enters at the positive terminal, passes through the heater M,

and out through the negative terminal. The filament B, which consists of a

short length of an infusible earth made of the oxides of several rare

minerals, of which zirconia is one, is a non-conductor at first, but

becomes a conductor upon being raised to a high temperature by means of the

heater M. As soon as the filament becomes conductive the current then

passes through the automatic cut-out H, and the armature D is attracted,

thus breaking the heater circuit. The current then flows from the positive

terminal {100} [Illustration] through the cut-out H, resistance J, and

filament B, and from thence out of the lamp. Since the resistance of the

filament decreases the hotter it gets, it is necessary to insert a

ballasting resistance in series with it which has the opposite property of

increasing its resistance as it gets hotter, to prevent the filament taking

too much current and destroying itself. Such a resistance, J, consists of a

filament of fine iron wire, which, to prevent oxidation from exposure to

the air, is enclosed in a glass bulb filled with hydrogen gas. Fig. 52_b_

shows the form of ballast resistance used in the small and large type of

lamp respectively.

Either direct or alternating current can be used with these lamps, and with

direct current the polarity must be strictly observed, and that the

positive wire is connected to the positive and the {101} negative wire to

the negative terminal. With the smaller type of lamp once it has been

correctly placed in its holder it is essential that it should not be

turned, as a change in the direction of the current will rapidly destroy

the filament.

[Illustration: FIG. 52c.]

The arrangement of the larger type of Nernst lamp can be readily seen from

the drawing, Fig. 52c.

Care must be taken to see that the voltage required by the burner and

resistance equals the voltage of the supply circuit, and that only parts of

the same amperage are used together on the same lamp. No advantage is

obtained by over-running a Nernst lamp, this only shortening its life

without increasing the light. Under normal conditions the average life of

the burner is about 700 hours.

The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75

watts per c.p. The light given is remarkably steady, and the lamps are

adaptable for all voltages from 100 to 300. In one of the large type of

lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at

215 volts, and the resistance 0.5 ampere at 20 volts, while one of the

smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts

and 0.25 ampere at 20 volts for the burner and resistance respectively. The

burner and heater are very fragile, and should never be handled except by

the porcelain plate to which they are attached. The lamps burn in air and

emit a brilliant white light of high actinic power, the intrinsic

brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with

1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon

filament lamps.

The chief advantage of the Nernst lamp from a photographic point of view

lies in the fact that it produces abundantly the blue and violet rays which

have the greatest chemical effect upon a photographic plate or film. These

rays are known as chemical or actinic rays, and are only slightly produced

in some types of incandescent electric lamps. Carbon-filament lamps are

very poor in this respect.

Because a light is visually brilliant it must by no means be assumed that

it is the best to use for purposes of photography, and this is a point over

which many photographers stumble when using artificial light. Many sources

of light, while excellent for illumination, have very low actinic powers,

while others may have low illuminating but high {103} actinic powers. A

lamp giving a light yellowish in colour has usually low actinic power,

while all those lamps giving a soft white light are generally found to be

highly actinic.

In addition to the actinic value of the source of illumination, the

photographic film used must be very carefully chosen, as the chemical

inertia of the sensitised film plays an important part in the successful

reproduction of the picture, and also, to a certain extent, affects the

speed of transmission. The length of exposure, the amount of light admitted

to the film, and the characteristics of the film itself, are all factors

which have a decided bearing upon the quality of the results obtained, and

the film found to be most suitable in one case will perhaps give very

unsatisfactory results in another.

In photo-telegraphy the length of exposure is determined by the time taken

by the transmitting stylus to trace over a conducting strip on the metal

print, and this time, of course, varies with the density of the image and

also with the speed of transmission.

The film in ordinary photography is chosen with regard to the subject and

the existing light conditions, and the amount of light admitted to the film

and the length of exposure are regulated accordingly. No such latitude is,

however, possible in photo-telegraphy. With each set of apparatus {104} the

various factors, such as the light value, the amount of light admitted to

the film, and the length of exposure, will be practically fixed quantities,

and the film that will give the most satisfactory results under these fixed

conditions can only be found by the rough-and-ready method of "trial and

error."

The films in common use are manufactured in four qualities, namely,

ordinary, studio, rapid, and extra rapid. These terms should really relate

to the light sensitiveness of the film (or, as it is technically termed,

the speed), but at the best they are a rough and very unsatisfactory guide,

for the reason that some unscrupulous makers, purely for business purposes,

do not hesitate to label their films and plates as slow, rapid, etc.,

without troubling to make any tests for correct classification.

The speed of photographic films and plates is generally indicated by a

number, and the system of standardisation adopted by the majority of makers

in this country is that originated by Messrs. Hurter & Driffield,

abbreviated H. & D. In their system the speed of the film and the exposure

varies in geometrical proportion, a film marked H. & D. 50 requiring double

the exposure of one marked H. & D. 100. The highest number always denotes

the highest speed, and the exposure varies inversely with the speed.

Besides the Hurter & Driffield method of {105} obtaining the speed numbers

of plates and films adopted by a large number of makers in this country,

there are also two standard English systems known as the W.P. No. (Watkin's

power number) and Wynne F. No., both of which are used to a fair extent.

The "Actinograph" number or speed number of a plate in the H. & D. system

is found by dividing 34 by a number known as the Inertia, the Inertia,

which is a measure of the insensitiveness of the plate, being determined

according to the directions laid down by Hurter & Driffield--that is, by

using pyro-soda developer and the straight portion only of the density

curve. If, for instance, the Inertia was found to be one-fifth, then the

speed number would be 34 / 1/5 = 170, and the plate is H. & D. 170. The

W.P. No. is found by dividing 50 by the Inertia. Thus 50 / 1/5 = 250, and

the plate is W.P. 250, but for all practical purposes the W.P. No. can be

taken as one and a half times H. & D. The Wynne F. numbers may be found by

multiplying the square root of the Watkins number by 6.4. Thus

[sqrt]250 = 15.81, and 15.81 x 6.4 = W.F. 101.

For those photographers who are in the habit of using an actinometer giving

the plate speeds in H. & D. numbers, the following table, taken from the

_Photographer's Daily Companion_, is given, {106} which shows at a glance

the relative speed numbers for the various systems. The Watkins and Wynne

numbers only hold good, however, when the inertia has been found by the H.

& D. method.

TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS

------------------------------------------------------

|H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.|

--------+--------+-----------------+--------+---------

| 10 | 15 | 24 || 220 | 323 | 114 |

| 20 | 30 | 28 || 240 | 352 | 120 |

| 40 | 60 | 49 || 260 | 382 | 124 |

| 80 | 120 | 69 || 280 | 412 | 129 |

| 100 | 147 | 77 || 300 | 441 | 134 |

| 120 | 176 | 84 || 320 | 470 | 138 |

| 140 | 206 | 91 || 340 | 500 | 142 |

| 160 | 235 | 103 || 380 | 558 | 150 |

| 200 | 294 | 109 || 400 | 588 | 154 |

------------------------------------------------------

Although theoretically the higher the speed of the film the less the

duration of exposure required, there is a practical limit, as besides the

intensity and actinic value of the light admitted to the film a definite

time is necessary for it to overcome the chemical inertia of the sensitised

coating and produce a useful effect. With every make of film it is possible

to give so short an exposure that although light does fall upon the film it

does no work at all--in other words, we can say that for every film there

is a minimum amount of light action, and anything below this is of no use.

The exposure that enables the smallest amount of light action to take place

is termed the limit of the smallest useful exposure. {107}

There is also a maximum exposure in which the light affects practically all

the silver in the film, and any increased light action has no increased

effect. This is the limit of the greatest useful exposure.

In photo-telegraphy the duration of exposure, as already pointed out, is

determined by certain conditions connected with the transmitting apparatus,

and with conditions similar to those mentioned on page 75 the length of

exposure will vary roughly from 1-50th to 1-150th of a second.

The most suitable film to use for purposes of photo-telegraphy is one

having a fairly slow speed in which the range of exposure required comes

well within the limits of the film. There is no advantage in using a film

having a speed of, say, H. & D. 300 if good results can be obtained from

one with a speed of, say, H. & D. 200, as the use of the higher speed

increases the risk of overexposure. With the high-speeded films the

difficulties of development are also greatly increased, there being more

latitude in both exposure and development with the slower speeds, and

consequently a better chance of obtaining a good negative.

Another point, often puzzling to the beginner, and which increases the

difficulty of choosing a suitable make of film, is that, although one make

of film marked H. & D. 100 will give good results, another make, also

marked H. & D. 100, will give {108} very poor results. This is owing, not

to a poor quality film, as many suppose, but to the almost insurmountable

difficulty of makers being able to employ exactly the same standard of

light for testing purposes, so that although various makes may all be

standardised by the H. & D. method, films bearing the same speed numbers

may vary in their actual speed by as much as 30 to 50 per cent.

* * * * *

{109}

APPENDIX A

SELENIUM CELLS

Selenium is a non-metallic element, and was first discovered by Berzelius

in 1817, in the deposit from sulphuric acid chambers, which still continues

the source from which it is obtained for commercial purposes, although it

is found to a small extent in native sulphur. Its at. wt. is 79.2, and its

sp. gr. 4.8. Symbol, Se.

In its natural state selenium is practically a non-conductor of

electricity, its resistance being forty thousand million times greater than

copper. Its practical value lies in the property which it possesses, that

when in a prepared condition it is capable of varying its electrical

resistance according to the amount of light to which it is exposed, the

resistance decreasing as the light increases.

Selenium is prepared by heating it to a temperature of 120deg C., keeping

it there for some hours, and allowing it to cool slowly, when it assumes a

crystalline form and changes from a bluish grey to a dull slate colour. A

selenium cell in its simplest form consists merely of some prepared

selenium placed between two or more metal electrodes, the selenium acting

as a high resistance conductor between them. The form given by Bell and

Tainter to the cells used in their experiments is given in Figs. 53 and

53a. It consists of a number of rectangular brass plates P, P', separated

by very thin sheets of mica M, the mica sheets being slightly narrower than

the brass plates, the whole being clamped together in the frame F by the

two bolts B. {110} By means of a sand-bath the cell is raised to the

desired temperature, and selenium is rubbed over the surface, which melts

and fills the small spaces between the brass plates. All the plates P are

connected together to form one terminal, and the plates P' to form the

other. By using very thin mica sheets, and a large number of elements, a

very narrow transverse section of selenium, together with a large active

surface, can be obtained.

The cell used for commercial purposes is usually constructed as follows. A

small rectangular piece of porcelain, slate, mica, or other insulator, is

wound with many turns of fine platinum wire. The wire is wound double, as

shown in Fig. 54, the spaces between the turns being filled with prepared

selenium. A thin glass cover is sometimes placed over the cell to protect

the surface from injury.

[Illustration: FIG. 53.

P, P', plates; M, mica; S, selenium.]

[Illustration: FIG. 53a.]

A strong light falling upon a cell lowers its resistance, and _vice versa_,

the resistance of a cell being at its highest when unexposed to light; the

light is apparently absorbed and made to do work by varying the electrical

resistance of the selenium. Selenium cells vary very considerably as

regards their quality as well as in their electrical resistance, it being

possible to obtain cells of the same size for any resistance between 10 and

1,000,000 ohms, and also, a cell may remain in good working condition for

several months, while another will become useless in as many weeks.

The ability of a cell to respond to very rapid changes in the illumination

to which it is exposed is determined largely upon its inertia, it being

taken as a general rule {111} that the higher the resistance of a cell the

less the inertia, and _vice versa_, and also, that the higher the

resistance the greater the ratio of sensitiveness. Inertia plays an

important part in the working of a cell, slightly opposing the drop in

resistance when illuminated, and opposing to a [Illustration] much greater

degree the return to normal for no-illumination. The effects of inertia or

"lag," as it is termed, can readily be seen by reference to Fig. 55. It

will be noticed that the current value rapidly increases when the cell is

first illuminated, but if after a short time _t_ the light is cut off, the

current value, instead of returning at once to normal for no-illumination,

only partially rises owing to the interference of the inertia, and some

time elapses before the cell returns to its normal condition; the time

varying from a few seconds to several minutes, depending upon the

characteristics of the cell and the amount of light to which it is exposed.

An actual curve is given in Fig. 55a. The inertia or "lag" of a cell

produces upon an intermittent current an effect similar to that produced by

the capacity [Illustration] of a line, as was noted in Chapter I.,

preventing the incoming signals from being recorded separately, and

distinctly. To obtain the best results in photo-telegraphy, the resistance

of a cell should only be decreased to an extent sufficient to pass the

current required to operate the recording apparatus, and the illumination

should be regulated so that this condition of the cell takes place.

The comparative slowness of selenium in responding to {112} any great

changes in the illumination offers a serious difficulty to its use in

photo-telegraphy, but various methods have been devised whereby the effects

of inertia can be counteracted. In the system of De' Bernochi (see Chapter

I.) the changes in the illumination are neither very rapid nor very great,

and the inertia effects would therefore be very slight; but in any

photo-telegraphic system in which a metal line print is used for

transmitting, where the source of illumination is constant and the

resistance of the cell is required to drop to a definite value and return

to normal instantly, many times in succession, the inertia effects are very

pronounced. The most successful method of counteracting the inertia is that

adopted by Professor Korn of always keeping the cell sufficiently

illuminated to overcome it, so that any additional light acts very rapidly.

Another method worked out and patented by Professor Korn, and known as the

"compensating cell" method, gives a practically dead beat action, the

resistance returning to its normal condition as soon as the illumination

ceases. The arrangement is given in the diagram Fig. 56.

[Illustration: FIG. 55a.]

Light from the transmitting or receiving apparatus, as the case may be,

falls upon the selenium cell S^1, which is {113} placed on one arm of a

Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The

selenium cell S^1 should have great sensitiveness and small inertia, the

compensating cell S^2 having proportionally small sensitiveness and large

inertia. Two batteries B, B', of about 100 volts, are connected as shown, B

being provided with a compensating variable resistance W; W' is also a

regulating resistance. When no light is falling upon the cell S^1, light

from L is prevented from reaching the second cell S^2 by a small shutter

which is fastened to the strings of the Einthoven galvanometer (described

in Chapter III.), and the piece of apparatus C--relay or galvanometer as

the case may be--remains in a normal condition. When, however, light falls

upon the cell S^1, the balance of the bridge is upset, and light from L

falls a fraction of a second later upon the second cell S^2, and the

current flowing through C completes the circuit. Needless to say it is

necessary that the two cells be well matched, as it is very easy to have

over-compensation, in which case the current is brought below zero.

[Illustration: FIG. 56.]

It is also stated that by enclosing the cells in exhausted glass tubes,

their inertia can be greatly reduced and their life considerably prolonged.

The sensitiveness of a cell is the ratio between its resistance in the dark

and its resistance when illuminated. The majority of cells have a ratio

between 2:1 and 3:1, but Professor Korn has shown mathematically that by

conforming to certain conditions regarding the construction the ratio of

sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms

can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only

a short distance away; the resistance may be still {114} further decreased

by continuing the illumination, but this produces a permanent defect in the

cells termed "fatigue," the cells becoming very sluggish in their action

and their sensitiveness gradually becoming less, the ratio between their

resistance in the dark and their resistance when illuminated being reduced

by as much as 30 per cent.

Excessive illumination will also produce similar results. The inertia of a

cell is practically unaffected by the wavelength of the light used, but the

maximum sensitiveness of a cell is towards the yellow-orange portion of the

spectrum.

In addition to light, heat has also been found to vary the electrical

resistance of selenium in a very remarkable manner. At 80deg C. selenium is

a non-conductor, but up to 210deg C. the conductivity gradually increases,

after which it again diminishes.

* * * * *

{115}

APPENDIX B

PREPARING THE METAL PRINTS

Electricians who desire to experiment in photo-telegraphy, but who have no

knowledge of photography, may perhaps find the following detailed

description of preparing the metal prints of some value. The would-be

experimenter may feel somewhat alarmed at the amount of work entailed, but

once the various operations are thoroughly grasped, and with a little

patience and practice, no very great difficulty should be experienced. The

simpler photographic operations, such as developing, fixing, etc., cannot

be described here, and the beginner is advised to study a good text-book on

the subject.

The method to be given of preparing the photographs is practically the only

one available for wireless transmission, and although the manner given of

preparing is perhaps not strictly professional, having been modified in

order to meet the requirements of the ordinary amateur experimenter, the

results obtained will be found perfectly satisfactory.

As will have been gathered from Chapter II., the camera used for copying

has to have a single line screen placed a certain distance in front of the

photographic plate, and the object of this screen is to break the image up

into parallel bands, each band varying in width according to the density of

the photograph from which it has been prepared. Thus a white portion of the

photograph would consist of very narrow lines wide apart, while a dark

portion would be made up of wide lines close together; a black part would

appear solid and show no lines at all. It is, of course, obvious {116} that

the lines on the negative cannot be wider apart, centre to centre, than the

lines of the screen. A good screen distance has been found to be 1 to 64,

_i.e._ the diameter of the stop is 1/64th of the camera extension, and the

distance of the screen lines from the photographic plate is 64 times the

size of the screen opening. The following table shows what this distance is

for the screen most likely to be used. The line screens used consist of

glass plates upon which a number of lines are accurately ruled, the width

of the lines and the spaces between being equal; the lines are filled in

with an opaque substance. These ruled screens are very expensive, and are

only made to order,[10] a screen half-plate size costing from 21s. to 27s.

6d. An efficient substitute for a ruled screen can be made by taking a

rather large sheet of Bristol board and ruling lines across in pure black

drawing ink, the width of the lines and the spaces between being 1/12th of

an inch respectively. A photograph must be taken of this card, the

reduction in size determining the number of lines to the inch. A card 20 x

15 inches, with 12 lines to the inch, would, if reduced to 5 x 4 inches,

make a screen having 48 lines to the inch. Preparing the board is rather a

tedious operation, but the line negative produced will be found to give

results almost as good as those obtained from a purchased screen.

DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.

--------------------------------------------------------------

|Screen ruling |Actual space| Distance of |In 1/32|In milli-|

|lines per inch.| in inches. |screen ruling| inches| metres.|

| | | in inches. | | |

|---------------+------------+-------------+-------+---------|

| 35 | 1/70 | .91 | 28.8 | 21.8 |

| 50 | 1/100 | .64 | 20.5 | 16.2 |

| 65 | 1/130 | .49 | 15.7 | 12.4 |

--------------------------------------------------------------

As it is impossible for many to have the use of professional apparatus

designed for this particular kind of work, {117} the fixing of the screen

into an ordinary camera must be left to the ingenuity of the worker. A

half-plate back focussing camera will be found suitable for general

experimental work, but if this is not available, a large box camera can be

pressed into service.

[Illustration: FIG. 57.]

The writer has never seen a half-plate box camera, but one taking a 5 x 4

inch plate can be obtained second-hand very cheaply. It is a comparatively

simple matter to fix the line screen into a camera of this description, the

drawings Figs. 57 and 58 showing the method adopted by the writer. The two

clips D, made from fairly stout brass about 1/2 inch wide, are bent to the

shape shown (an enlarged section is given at C) and soldered at the top and

bottom of one of the metal sheaths provided for holding the plates. The

distance between the front of the photographic plate (the film side) and

the back of the line screen (also the film side), indicated by the arrow at

A, is determined by the number of lines on the screen. As will be seen from

the table given, the distance for a screen having 50 lines to the inch will

be 41/64ths of an inch.

[Illustration: FIG. 58.

M, sheath; P, photographic plate; D, clips; S, line screen.]

In all probability there will be enough clearance between the top of the

sheath and the top of the camera to allow for the thickness of the clip,

but if not, a shallow groove a little wider than the clip should be

carefully cut in the top of the camera, so that it will slide in easily.

The screen should be placed between the clips, the film side on the {118}

inside, _i.e._ facing the photographic plate. As with a box camera the

extension is a fixture, the size of stop to be used is a fixture also. The

extension of a camera (this term really applies to a bellows camera) is

measured from the front of the photographic plate to the diaphragm, and if

this distance in our camera is 8 inches, then the diameter of the stop to

give the best results would be 1/64th of this, or 1/8th inch. Although for

all ordinary experimental work the lens fitted to the camera will be

suitable, the best type of lens for process work of all kinds is the

"Anastigmat."

The picture or photograph from which it is desired to make a print should

be fastened out perfectly flat upon a board with drawing pins, and if a

copying stand is not available it must be placed upright in some convenient

position. The diagram Fig. 59 gives the disposition of the apparatus

required for copying. A simple and inexpensive copying stand is shown in

Fig. 60. The blackboard A should be about 30 inches square, and must be

fastened perfectly upright upon the base-board B. The stand C should be

made so that it slides without any side play between the guides D, and

should be of such a height that the lens of the camera comes exactly

opposite the {119} [Illustration] [Illustration] centre of the board A. The

camera, if of the box type, can be secured to the stand by means of a screw

and wingnut, the screw being passed from the inside as shown. The beginner

is advised to photograph only very bold and simple subjects, such as black

and white drawings or enlargements. It is not safe to trust to the

view-finders as to whether the whole of the picture is included on the

plate, a piece of ground glass the same size as the plate sheaths, and used

as a focussing screen, being much more reliable. It is a good plan to focus

the camera for a number of different-sized pictures, marking the board A,

and the {120} guides D, so that adjustment is afterwards a very simple

matter.

The make of plate used is also a great factor in getting a good negative,

and Wratten Process Plates will be found excellent. As already mentioned,

such subjects as the exposure and the development of the plate cannot be

dealt with here, these subjects having been exhaustively treated in several

text-books on photography. With an arc lamp the exposure is about twice as

long as in daylight, but the exposure varies with the amount of light

admitted to the plate, character of the source of light, and the

sensitiveness of the plate used, etc. The writer has used acetylene gas

lamps for this purpose with great success. The beginner is advised to use

artificial light, as this can be kept perfectly even. With daylight,

however, the light is constantly fluctuating, and this renders the use of

an actinometer a necessity for correct exposure. After development, if the

plate is required for immediate use, it can be quickly dried by soaking for

a few minutes in methylated spirit.

Having obtained a good negative, the next operation is to prepare what is

known as a metal print. For this we shall require some stout tin-foil or

lead-foil, about 12 or 15 square feet to the pound, and this should be cut

into pieces of such a size that it allows a lap of 3/16 inch when wrapped

round the drum of the transmitting machine. Obtain some good fish-glue and

add a saturated solution of bichromate of potash in the proportion of 4

parts of potash to 40 or 50 parts of glue. Pour a little of this glue into

a shallow dish, lay a sheet of foil upon a flat board, and with a fairly

stiff brush (a flat hog's-hair as wide as possible) proceed to coat the

sheet of foil with a thin but perfectly even coating of glue. The thickness

of the coating can only be found by trial, for if the coating is too thick

a longer time will be required for printing; but it must not be thin enough

to show interference colours. After the coating has been laid on, a soft

brush, such as photographers use for dusting dry {121} plates, should be

passed up and down, and across and across, with light, even strokes to

remove any unevenness. A glue solution used by professional photo-engravers

is as follows:

Fish-glue 12 oz.

Bichromate of Ammonia 3/4 oz.

Water 18 to 24 oz.

Ammonia .880 30 minims.

The bichromate should be dissolved in the water, and, when added to the

glue, stir very thoroughly in order that complete mixing may take place.

The coating may be done in a good light, not bright sunlight, but _it must

be dried in the dark_, because, although insensitive while in a moist

condition, it becomes sensitive immediately on desiccation. If allowed to

dry in the light the whole coating will become insoluble, and for this

reason the brushes used should be washed out as soon as they are finished

with. The sheets will take about 15 minutes to dry in a perfectly dry room,

but it is not advisable to prepare many sheets at once, as they will not

keep for more than two or three days.

The prepared negative must now be placed in an ordinary printing frame, and

a print taken off upon one of the metal sheets in the same way as a print

is taken off upon ordinary sensitised paper. In daylight the exposure

varies from 5 to 20 minutes, but in artificial light various trials will

have to be made in order to get the best results, the exposure varying with

the amount of bichromate in the coating; the proportion of the bichromate

to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp

for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to

"print" the impression on the metal sheets. The printing finished, the

metal print should be laid upon a sheet of glass and held under a running

stream of water. The washing is complete as soon as the unexposed parts of

the glue coating have been entirely washed away leaving the bare metal, and

this will take anything from 3 to 7 {122} minutes, depending upon the

thickness of the film. As soon as it is dry the print is ready for use.

As already mentioned, the negative from which the metal print is made

requires that the lines be perfectly sharp and opaque, and the spaces

between perfectly transparent. Ordinary dry plates are too rapid, a rather

slow plate being required. Wratten Process Plates give excellent results,

and the following is a good developer to use with them:

Glycin 15 grammes 1 oz.

Sulphite of Soda 40 ,, 2-1/2 ,,

Carbonate of Potash 80 ,, 5 ,,

Water 1000 c.c. 60 ,,

This developer should be used for 6 minutes at a temperature of 50deg F.,

3-1/2 minutes at 65deg, and 1-3/4 minutes at 80deg. It is best only used

once. If an intensifier is required, the following formula will be found to

give satisfactory results:

Bichloride of Mercury 1 oz. 60 grammes.

Hot Water 16 ,, 1000 c.c.

Allow to cool, completely pour off from any crystals, and add:

Hydrochloric Acid 30 minims 4 c.c.

Allow negative to bleach thoroughly, wash well in water, and blacken in 10

per cent ammonia .880, or 5 per cent sodium sulphide.

In preparing the negatives and metal prints the following points should be

observed:

A good negative should have the lines perfectly sharp and opaque; there

should be no "fluff" between the lines even when they are close together.

A properly exposed and developed negative should not require any reducing

or intensifying.

If the lamps used for illuminating the copying board are placed 2 feet

away, and the exposure required is 5 minutes, the exposure, if the lamps

are placed 4 feet away, will be {123} 20 minutes, as the amount of light

which falls upon an object decreases as the inverse square of the distance.

Get the coating on the foil as thin as possible, and err on the side of

over-exposure, for if the coating is thick and has been under-exposed,

excessive washing will dissolve the whole coating; for, unless

insolubilisation has taken place right up to the metal base, the under

parts will remain in a more or less soluble condition.

On no account must the unexposed sheets be placed near a fire, otherwise

they will be spoilt, the whole coating becoming insoluble; heat acting in

the same manner as light.

In washing, keep the print moving so that the stream of water does not fall

continually in one place. It is best to hold the print so that the water

runs off in the direction of the lines.

To dry the prints after washing they can be laid out flat in a moderately

warm oven, or before a stove, the heat of course not being sufficient to

cause the coating to peel.

To render the glue image more distinct the print should be immersed for a

few seconds in an aniline dye solution, the glue taking up the colour

readily. These dyes are soluble in either water or alcohol. A dye known as

"magenta" is very good.

The process of coating the metal sheets must be performed as quickly as

possible (about 10 seconds), as owing to the peculiar nature of the

bichromated glue it soon sets, and once this has taken place it is

impossible to smooth down any unevenness.

See that the negative and metal sheet make good contact while printing.

If the glue solution does not adhere to the surface of the foil in a

perfectly even film, but assumes a streaky appearance, a little liquid

ammonia, or a weak solution of nitric acid, rubbed over the surface of the

foil, which is afterwards gently scoured with precipitated chalk on a tuft

of cotton {124} wool, will remove the grease which is the cause of the

difficulty.

A photograph of a picture prepared from a line negative is given in Fig.

61. For a great many experiments, and in order to save time, trouble, and

expense, sketches drawn upon stout lead-foil in an insulating ink will

answer the purpose admirably, but if any exact work is to be done a single

line print is of course absolutely necessary. The insulating ink can be

prepared by dissolving shellac in methylated spirit, or ordinary gum can be

used. A very fine brush should be used in place of a pen, as the gum will

not flow freely from an ordinary nib unless greater pressure than the foil

will safely stand be applied. A sketch prepared in this manner is shown in

Fig. 62. A little aniline dye should be added to the gum to render it more

visible, or a mixture of gum and liquid indian ink will be found suitable.

[Illustration: FIG. 63.]

With the copying arrangement already described it is only possible to

employ it for reducing, it being necessary to employ a bellows camera with

a back focussing attachment for purposes of enlarging, and this constitutes

the chief drawback to the use of a fixed focus camera. By replacing the box

camera with a focussing camera of the same size, we shall have a piece of

apparatus capable of reducing or enlarging, only in this case the camera

should be a fixture and the board, A, arranged to slide backwards and

forwards instead.

[Illustration: FIG. 61.

Portions of photographs (full size) of single line screen, and single line

print. Screen 40 lines to the inch.]

[Illustration: FIG. 62.]

{125} An extra improvement would be to rule the surface of the copying

board, A, in a manner similar to that shown in the diagram, Fig. 63. The

rulings should be marked off from the centre of the board, and should

enclose parallelograms of the various plate sizes ranging from 3-1/4 x

4-1/4 inches up to the full size of the board. By fastening the picture or

photograph to be copied in the space on the board corresponding in size, we

can ensure that it is in the correct position for the whole to be included

on the photographic plate, providing, of course, that the centre of lens

and board coincide.

With regard to the lens required, the practice adhered to by most

photographers is to use a lens having a focal length equal to the diagonal

of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be

used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 x 4 inch

camera a 6-inch lens will be required. The following is a simple rule for

finding the conjugate foci of a lens, and is useful in obtaining the

distance from the lens to the photographic plate and the picture to be

copied. Let us suppose that we wish to make a 1-1/2 times enlarged line

negative from a 4-1/4 x 3-1/4 inch print. Add 1 to the number of times it

is required to enlarge and multiply the result by the focal length of the

lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if

a 6-inch lens is used, 2-1/2 x 6 = 15 inches will be the distance of the

lens from the plate. Divide this number by the number of times it is

desired to enlarge, and the distance of the lens from the picture to be

copied is obtained; in this instance 15 / 1-1/2 = 10 inches. The same rule

can be followed when it is required to reduce any given number of times,

only in this case the greater number will represent the distance between

the lens and the picture to be copied, and the lesser number the distance

between the lens and the plate.

In reducing, a 1/4-plate lens will be found to fully cover a 5 x 4 inch

plate, providing the reduction is not greater than three to one.

* * * * *

{126}

APPENDIX C

LENSES

In this small volume it is not desirable, neither is it intended, to give

an exhaustive treatment on the subject of lenses and their action, but as

optics plays an important part in the transmission of photographs, both by

wireless and over ordinary conductors, the following notes relating to a

few necessary principles have been included as likely to prove of interest.

Light always travels in straight lines when in a medium of uniform density,

such as water, air, glass, etc., but on passing from one medium to another,

such as from air to water, or air to glass, the direction of the light rays

is changed, or, to use the correct term, _refracted_. This refraction of

the rays of light only takes place when the incident rays are passed

obliquely; if the incident rays are perpendicular to the surface separating

the two media they are not refracted, but continue their course in a

straight line.

All liquid and solid bodies that are sufficiently transparent to allow

light rays to pass through them possess the power of bending or refracting

the rays, the degree of refraction, as already explained, depending upon

the nature of the body.

The law relating to refraction will perhaps be better understood by means

of the following diagram. In Fig. 64 let the line AB represent the surface

of a vessel of water. The line CD, which is perpendicular to the surface of

the {127} water, is termed the _normal_, and a ray of light passed in this

direction will continue in a straight line to the point E. If, however, the

ray is passed in an oblique direction, such as ND, it will be seen that the

ray is bent or refracted in the direction DM; if the ray of light is passed

in any other oblique direction, such as JD, the refracted ray will be in

the direction DK. The angle NDC is called the _angle of incidence_ and MDE

the _angle of refraction_. If we measure accurately the line NC, we shall

find that it is 1-1/3, or more exactly 1.336, times greater than the line

EM. If we repeat this measurement with the lines JH and PK we shall find

that the line JH also bears the proportion of 1.336 to the line PK. The

line NC is called the _sine of the angle of incidence_ NDC, and EM the

_sine of the angle of refraction_ MDE.

[Illustration: FIG. 64.]

Therefore in water the sine of the angle of incidence is to the sine of the

angle of refraction as 1.336 is to 1, and this is true whatever the

position of the incident ray with respect to the surface of the water. From

this we say that _the sines of the angles of incidence and refraction have

a constant proportion or ratio to one another_.

The number 1.336 is termed the _refractive index_, or _coefficient_, or the

_refractive power_ of water. The refractive power varies, however, with

other fluids and solids, and a complete table will be found in any good

work on optics.

Glass is the substance most commonly used for refracting the rays of light

in optical work, the glass being worked up into different forms according

to the purpose for which it {128} is intended. Solids formed in this way

are termed _lenses_. A lens can be defined as a transparent medium which,

owing to the curvature of its surfaces, is capable of converging or

diverging the rays of light passed through it. According to its curvature

it is either spherical, cylindrical, elliptical, or parabolic. The lenses

used in optics are always exclusively spherical, the glass used in their

construction being either crown glass, which is free from lead, or flint

glass, which contains lead and is more refractive than crown glass. The

refractive power of crown glass is from 1.534 to 1.525, and of flint glass

from 1.625 to 1.590. Spherical surfaces in combination with each other or

with plane surfaces give rise to six different forms of lenses, sections of

which are given in Fig. 65.

[Illustration: FIG. 65.]

All lenses can be divided into two classes, convex or converging, or

concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses,

being thicker at the middle than at the borders, and _d_, _e_, _f_, which

are thinner at the middle, being diverging lenses. The lenses _e_ and _g_

are also termed meniscus lenses, and _a_ represents a prism. The line XY is

the axis or _normal_ of these lenses to which their plane surfaces are

perpendicular.

Let us first of all notice the action of a ray of light when passed through

a prism. The prism, Fig. 66, is represented by the triangle BBB, and the

incident ray by the line TA. {129} Where it enters the prism at A its

direction is changed and it is bent or refracted towards the base of the

prism, or towards the normal, this being always the case when light passes

from a rare medium to a dense one, and where the light leaves the opposite

face of the prism at D it is again refracted, but away from the normal in

an opposite direction to the incident ray, since it is passing from a dense

to a rare medium. The line DP is called the _emergent_ or refracted ray. If

the eye is placed at T, and a bright object at P, the object is seen not at

P, but at the point H, since the eye cannot follow the course taken by the

refracted rays. In other words, objects viewed through a prism always

appear deflected towards its summit.

[Illustration: FIG. 66.]

In considering the action of a lens we can regard any lens as being built

up of a number of prisms with curved faces in contact. Such a lens is shown

in Fig. 67, the light rays being refracted towards the base of the prisms

or towards the normal, as already explained; while the top half of the lens

will refract all the light downwards, the bottom half will act as a series

of inverted prisms and refract all the light upwards.

[Illustration: FIG. 67.]

[Illustration: FIG. 68.]

If a beam of parallel light--such as light from the sun--be passed through

a double convex lens L, Fig. 68, we shall find that the rays have been

refracted from their parallel course and brought together at a point F.

This point F is {130} termed the principal focus of the lens, and its

distance from the lens is known as the focal length of that lens. In a

double and equally convex lens of glass the focal length is equal to the

radius of the spherical surfaces of the lens. If the lens is a plano-convex

the focal length is twice the radius of its spherical surfaces. If the lens

is unequally convex the focal length is found by the following rule:

multiply the two radii of its surfaces and divide twice that product by the

sum of the two radii, and the quotient will {131} be the focal length

required. Conversely, by placing a source of light at the point F the rays

will be projected in a parallel beam the same diameter as the lens. If,

however, instead of being parallel, the rays proceed from a point farther

from the lens than the principal focus, as at A, Fig. 69, they are termed

divergent rays, but they also will be brought to a focus at the other side

of the lens at the point a. If the source of light A is moved nearer to the

principal focus of the lens to a point A^1 the rays will come to a focus at

the point _a_^1, and similarly when the light is at A^2 the rays will come

to a focus at the point _a_^2. It can be found by direct experiment that

the distance _fa_ increases in the same proportion as AF diminishes, and

diminishes in the same proportion as AF increases. The relationship which

exists between pairs of points in this manner is termed the _conjugate

foci_ of a lens, and though every lens has only one principal focus, yet

its conjugate foci are innumerable.

[Illustration: FIG. 69.]

The formation of an image of some distant object in its principal focus is

one of the most useful properties of a convex lens, and it is this property

that forms the basis of several well-known optical instruments, including

the camera, telescope, microscope, etc.

If we take an oblong wooden box, AA, and substitute a sheet of ground

glass, C, for one end, and drill a small pinhole, H, in the centre of the

other end opposite the {132} glass plate, we shall find that a tolerably

good image of any object placed in front of the box will be formed upon the

glass plate. The light rays from all points of the object, BD, Fig. 70,

will pass straight through the hole H, and illuminate the ground glass

screen at points immediately opposite them, forming a faint inverted image

of the object BD. The purpose of the hole H is to prevent the rays from any

one point of the object from falling upon any other point on the glass

screen than the point immediately opposite to it, therefore the smaller we

make H, the more distinct will be the image obtained. Reducing the size of

H in order to produce a more distinct image has the effect of causing the

image to become very faint, as the smaller the hole in H, the smaller the

number of rays that can pass through from any point of the object. By

enlarging the hole H gradually, the image will become more and more

indistinct until such a size is reached that it disappears altogether.

[Illustration: FIG. 70.]

If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose

focal length suits the length of the box, the image produced will be

brighter and more distinct than that formed by the aperture, H, since the

rays which proceed from any point of the object will be brought by the lens

to a focus on the glass screen, forming a bright {133} distinct image of

the point from which they come. The image owes its increased distinctness

to the fact that the rays from any one point of the object cannot interfere

with the rays from any other point, and its increased brightness to the

great number of rays that are collected by the lens from each point of the

object and focussed in the corresponding point of the image. It will be

evident from a study of Fig. 71 that the image formed by a convex lens must

necessarily be inverted, since it is impossible for the rays from the end,

M, of the object to be carried by refraction to the upper end of the image

at _n_. The relative positions of the object and image when placed at

different distances from the lens are exactly the same as the conjugate

foci of light rays as shown in Fig. 69.

[Illustration: FIG. 71.]

The length of the image formed by a convex lens is to the length of the

object as the distance of the image is to the distance of the object from

the lens. For example, if a lens having a focal length of 12 inches is

placed at a distance of 1000 feet from some object, then the size of the

image will be to that of the object as 12 inches to 1000 feet, or 1000

times smaller than the object; and if the length of the object is 500

inches, then the length of the image will be the 1/1000th part of 500

inches, or 1/2 inch. {134}

The image formed by the convex lens in Fig. 71 is known as a _real image_,

but in addition convex lenses possess the property of forming what are

termed _virtual images_. The distinction can be expressed by saying, _real

images are those formed by the refracted rays themselves, and virtual

images those formed by their prolongations_. While a real image formed by a

convex lens is always inverted and smaller than the object, the virtual

image is always erect and larger than the object. The power possessed by

convex lenses of forming virtual images is made use of in that useful but

common piece of apparatus known as a reading or magnifying glass, by which

objects placed within its focus are made larger or magnified when viewed

through it; but in order to properly understand how objects seem to be

brought nearer and apparently increased in size, we must first of all

understand what is meant by the expression, _the apparent magnitude of

objects_.

[Illustration: FIG. 72.]

The apparent magnitude of an object depends upon the angle which it

subtends to the eye of the observer. The image at A, Fig. 72, presents a

smaller angle to the eye than the angle presented by the object when moved

to B, and the image therefore appears smaller. When the object is moved to

either B or C, it is viewed under a much {135} greater angle, causing the

image to appear much larger. If we take a watch or other small circular

object and place it at A, which we will suppose is a distance of 50 yards,

we shall find that it will be only visible as a circular object, and its

apparent magnitude or the angle under which it is viewed is then stated to

be very small. If the object is now moved to the point B, which is only 5

feet from the eye, its apparent magnitude will be found to have increased

to such an extent that we can distinguish not only its shape, but also some

of the marking. When moved to within a few inches from the eye as at C, we

see it under an angle so great that all the detail can be distinctly seen.

By having brought the object nearer the eye, thus rendering all its parts

clearly visible, we have actually magnified it, or made it appear larger,

although its actual size remains exactly the same. When the distance

between the object and the observer is known, the apparent magnitude of the

object varies inversely as the distance from the observer.

Let us suppose that we wish to produce an image of a tree situated at a

distance of 5000 feet. At this distance the light rays from the tree will

be nearly parallel, so that if a lens having a focal length of 5 feet is

fastened in any convenient manner in the wall of a darkened room the image

will be formed 5 feet behind the lens at its principal focus. If a screen

of white cardboard be placed at this point we shall find that a small but

inverted image of the tree will be focussed upon it. As the distance of the

object is 5000 feet, and as the size of the received image is in proportion

to this distance divided by the focal length of the lens, the image will be

as 5000 / 5, or 1000 times smaller than the object.

If now the eye is placed six inches behind the screen and the screen

removed, so that we can view the small image distinctly in the air, we

shall see it with an apparent magnitude as much greater than if the same

small image were equally far off with the tree, as 6 inches is to 5000

{136} feet, that is 10,000 times. Thus we see that although the image

produced on the screen is 1000 times less than the tree from one cause, yet

on account of it being brought near to the eye it is 10,000 times greater

in apparent magnitude; therefore its apparent magnitude is increased as

10,000 / 1000, or 10 times. This means that by means of the lens it has

actually been magnified 10 times. This magnifying power of a lens is always

equal to the focal length divided by the distance at which we see small

objects most distinctly, viz. 6 inches, and in the present instance is 60 /

6, or 10 times.

When the image is received upon a screen the apparatus is called a _camera

obscura_, but when the eye is used and sees the inverted image in the air,

then the apparatus is termed a _telescope_.

The image formed by a convex lens can be regarded as a new object, and if a

second lens is placed behind it a second image will be formed in the same

manner as if the first image were a real object. A succession of images can

thus be formed by convex lenses, the last image being always treated as a

fresh object, and being always an inverted image of the one before. From

this it will be evident that additional magnifying power can be given to

our telescope with one lens by bringing the image nearer the eye, and this

is accomplished by placing a short focus lens between the image and the

eye. By using a lens having a focal length of 1 inch, and such a lens will

magnify 6 times, the total magnifying power of the two lenses will be 10 x

6 = 60 times, or 10 times by the first lens and 6 times by the second. Such

an instrument is known as a _compound or astronomical telescope_, and the

first lens is called the object glass and the second lens the magnifying

glass, or eye-piece.

We are now in a position to understand how virtual images are formed, and

the formation of a virtual image by means of a convex lens will be readily

followed from a {137} study of Fig. 73. Let L represent a double convex

lens, with an object, AB, placed between it and the point F, which is the

principal focus of the lens. The rays from the object AB are refracted on

passing through the lens, and again refracted on leaving the lens, so that

an image of the object is formed at the eye, N. As it is impossible for the

eye to follow the bent rays from the object, a virtual image is formed and

is seen at A^1B^1, and is really a continuation of the emergent rays. The

magnifying power of such a lens may be found by dividing 6 inches by the

focal length of the lens, 6 inches being the distance at which we see small

objects most distinctly. A lens having a focal length of 1/4 inch would

magnify 24 times, and one with a focal length of 1/100th of an inch 600

times, and so on. The magnifying power is greater as the lens is more

convex and the object near to the principal focus. When a single lens is

applied in this manner it is termed a _single microscope_, but when more

than one lens is employed in order to increase the magnifying power, as in

the telescope, then the apparatus is termed a _compound microscope_.

[Illustration: FIG. 73.]

Unlike a convex lens, which can form both real and virtual images, a

concave lens can only produce a virtual image; and while the convex lens

forms an image larger {138} than the object, the concave lens forms an

image smaller than the object. Let L, Fig. 74, represent a double concave

lens, and AB the object. The rays from AB on passing through the lens are

refracted, and they diverge in the direction RRRR, as if they proceeded

from the point F, which is the principal focus of the lens, and the

prolongations of these divergent rays produce a virtual image, erect and

smaller than the object, at A^1B^1. The principal focal distance of concave

lenses is found by exactly the same rule as that given for convex lenses.

[Illustration: FIG. 74.]

Up to the present we have assumed that all the rays of light passed through

a convex lens were brought to a focus at a point common to all the rays,

but this is really only the case with a lens whose aperture does not exceed

12deg. By aperture is meant the angle obtained by joining the edges of a

lens with the principal focus. With lenses having a larger aperture the

amount of refraction is greater at the edges than at the centre, and

consequently the rays that pass through the edges of the lens are brought

to a focus nearer the lens than the rays that pass through the centre.

Since this defect arises from the spherical form of the lens it is termed

_spherical aberration_, and in lenses that {139} are used for photographic

purposes the aberration has to be very carefully corrected.

The distortion of an image formed by a convex lens is shown by the diagram,

Fig. 75. If we receive the image upon a sheet of white cardboard placed at

A, we shall find that while the outside edges will be clear and distinct,

the inside will be blurred, the reverse being the case when the cardboard

is moved to the point B.

[Illustration: FIG. 75.]

[Illustration: FIG. 76.]

[Illustration: FIG. 77.]

Aberration is to a great extent minimised by giving to the lens a meniscus

instead of a biconvex form, but as it is desirable to reduce the aberration

to below once the {140} thickness of the lens, and as this cannot be done

by a single lens, we must have recourse to two lenses put together. The

thickness of a lens is the difference between its thickness at the middle

and at the circumference. In a double convex lens with equal convexities

the aberration is 1-67/100ths of its thickness. In a plano-convex lens with

the plane side turned towards parallel rays the aberration is 4-1/2 times

its thickness, but with the convex side turned towards parallel rays the

aberration is only 1-17/100ths of its thickness.

By making use of two plano-convex lenses placed together as at Fig. 76, the

aberration will be one-fourth of that of a single lens, but the focal

length of the lens, L^1, must be half as much again as that of L. If their

focal lengths are equal the aberration will only be a little more than half

reduced. Spherical aberration, however, may be entirely destroyed by

combining a meniscus and double convex lens, as shown in Fig. 77, the

convex side being turned to the eye when used as a lens, and to parallel

rays when used as a burning glass or condenser.

* * * * *

{141}

INDEX

Aberration, 139

spherical, 138, 140

Accuracy of working, 70, 72

Acetylene gas lamps, 120

Actinic power, 102

Actinograph, 105

Actinometer, 120

Alternating current, 82, 100

Ammonia, 123

Angle of stylus, 24, 78

Aniline dye, 123

Arcing, 27, 82

Arc lamps, 15, 120, 121

Atmospherics, 61, 85

Ballasting resistance, 100

Belin, 47

Bernochi, 7, 112

system of, 7, 34

Berzelius, 109

Bichromate of potash, 120

Blondel's oscillograph, 47

Camera obscura, 136

extension, 116, 118

choice of, 117

Capacity of condenser, 24, 78

electrostatic, 3, 5

of cable, 3

of London-Paris telephone line, 3

Carbon bisulphide, 53

Charbonelle, 48

receiver of, 48

Chemical solution, 56

Circuit breaker, 76

Clutch, details of, 88, 89, 91

spring, 71

Coating the metal sheets, 120

Coherer, 11, 40

Collecting rings, 91

Commercial value of photo-telegraphy, 1

Compensating selenium cell, 112

Contact breaker, 37

Copying arrangements, 118, 125

Cross screen, 21

De' Arsonval galvanometer, 47, 73

Decoherer, 41

Design of machines, 21

Detectors, 83

Developing solutions, 105, 122

Diaphragm, movement of, 48, 52, 84, 87

Dipping rods, 81, 83

Distance of transmission, 33

Duration of wave-trains, 22, 25

Early experiments, 2

Einthoven galvanometer, 32, 44, 45, 54, 113

Electric clock, 93

Electrolytic receiver, 4, 37, 54, 61, 64

Enlarging arrangements, 124, 125

Experimental machine, 20

Extraneous light, 47

Fastening electrolytic paper, 58

Fatigue of selenium cell, 64, 114

Fish glue, 120

Flexible couplings, 77

Frequency meter, 65

Friction brake, 88

{142}

High speed telegraphy, 70

Hughes governor, 65

Hughes printing telegraph, 63

Hurter and Driffield, 104

Hydrogen, 100

Incidence, angle of, 127

Inertia, 64, 65, 111

effects in photo-telegraphy, 110

method of counteracting, 103, 112, 113

effect of wave-length of light on, 114

Intensifying solution, 122

Isochroniser, 89, 91

details of, 91, 92, 95

Isochronism, 64, 69, 70, 71

Kathode rays, 53

Knudsen, 2

apparatus of, 9

Korn, 30, 33, 45, 65, 72

apparatus of, 31

Lamps, coloured, 94

Lenses, 85, 125, 128

principal focus of, 130

conjugate foci of, 131

action of, 129

convex, 128, 131, 136

concave, 128, 138

focal length of, 130, 138

aperture, 138

meniscus, 139

Light, diffusion of, 86

extraneous, 87

Limit of error in synchronising, 64

Line balancer, 3

Line screens, 9, 15, 16, 116

making, 116

Magnifying power, 136, 137

Marconi valve, 44, 54

coherer, 40

Mechanical inertia, 33

Mercury break, 81

churning of, 82

containers, 82

Mercury jet interrupter, 29

Metal prints, 15, 18, 32, 59, 64, 95, 120, 124

drying the, 121, 123

exposure of, 121

size of, 22, 24, 75, 77

pressing the, 22

Microscope, 131, 137

Military uses, 35

Mirror galvanometer, 9, 42, 73

Mirror, 47, 51

Morse code, 35

Motor speed, 89, 95

driving, 91, 93, 95

clockwork, 63

electric, 63

Nernst lamps, 43, 85, 98

heater of, 99

filament of, 99

principle of, 98

resistance of, 100

efficiency of, 101, 102

overrunning, 101

Nicol prism, 53

Paper for electrolytic receiver, 56

Parabolic reflector, 8

Period of galvanometer, 43, 44, 46

_Photographic Daily Companion_, 105

Photographic films, 40, 43, 45, 53, 54, 62, 85, 86, 98

process, 37

chemical inertia, 103

exposure of, 103, 107

speed of, 104, 105

plates, orthochromatic, 59

plates, 120

Points to be observed in preparing metal prints, 123

Poulsen Company, 32, 47

arc, 31

Preparing selenium, 109

photographs for transmitting, 15, 115

sketches on metal foil, 124

Prism, 128

action of, 129

Process plates, 122

Professor Nernst, 98

{143}

Radio-photography, requirements of, 74

Refraction, angle of, 127

Refractive power, 127

Relay, 25, 39, 49, 53, 55, 60, 75

differential, 79

polarised, 97

working speed of, 26, 75

Reproducing for newspapers, 60

Resistance of selenium, 109

of selenium cells, 110

regulating, 113

Retardation of current, 6

Retouching, 62

Rotary spark-gap, 28

Selenium, 99

cells, 8, 34, 55, 60, 64, 109, 110

machines, 45

Self-induction, 24, 78

Sensitiveness of selenium cells, 113

ratio of, 113

Silvered quartz threads, 44, 46

Spark-gap, 27

Speed regulator, 68

adjustments of, 69

Spring clutch, 71

Starting position of machines, 98

String galvanometer, 32

Stylus, 17, 18, 57, 61, 78, 95, 103

sparking at, 24

Stylus, angle of, 24, 78

defects of, 57

Submarine cable, 4

Synchronism, 11, 20, 36, 64, 69, 71

Telephograph, 74

advantages of, 76

method of working, 96

Telephone receiver, 83, 85

diaphragm, 48

improved, 51

Telephone relay, 48, 50, 52, 83, 85, 97

Telescope, 131, 136

Thermodetector, 32

Tow, 88

Transmission, distance of, 35, 72

speed of, 25, 35, 75

Vibration, natural period of, 39

Watkins, 105

power number, 105

Waves, damped, 30

undamped, 30, 31

Wheatstone bridge, 113

Wireless apparatus, 13

_Wireless World_, 31

Wynne, 105

Zirconia, 99

THE END

_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.

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Notes

[1] These measurements only apply to a single line. Where a double line is

employed the capacity is halved.

[2] See Appendix A.

[3] See Appendix B.

[4] In wireless telegraphy "arcing" is principally caused by the

continuation of the supply current in the spark-gap after the capacity has

been charged to a potential sufficient to break down the insulation of the

gap.

[5] See Chapter V.

[6] Nernst lamps are the best to use, as they produce abundantly the blue

and violet rays which have the greatest chemical effect upon a photographic

film. Carbon filament lamps are very poor in this respect.

[7] A description of the apparatus required will be found in Ganot's

_Physics_.

[8] Great care must be exercised in using this solution, as it is

exceedingly poisonous.

[9] Two clocks would isochronise if their hands travelled at precisely the

same rate round the dials, but would not synchronise unless they both

registered the same time as well.

[10] Line screens can be obtained from Messrs. Penrose, 109 Farringdon

Street, London; or Messrs. Fallowfield, 146 Charing Cross Road, London.