the most brilliant of the spectrum, and of which the dark and light
stripes consequently coincide with the darkest and brightest stripes
of the fringes produced by white light, which is commonly employed
in these experiments, both because of its greater brightness, and
because of the more marked character which it gives to the central
stripe, so as to prevent any other from being mistaken for it.
It was an apparatus of this kind that Mr. ARAGO and myself employed
for measuring the difference of the refractive powers of dry air, and
of air saturated with moisture at 80° F., which is so small, that it
would escape every other method of observation, because the greater
refractive power of aqueous vapour is almost exactly compensated
by the less specific gravity of moist air. But, in the generality
of cases, the slightest mixture of one vapour or gas with another
produces a considerable displacement in the fringes: and if we had
a series of experiments of this kind, made with care, the apparatus
might become a valuable instrument of chemical analysis.
[To be continued.]
iii. _Remarks on the Action of_ CORPUSCULAR FORCES. _In a Letter to_
Mr. POISSON.
My dear Sir,
I am very glad to see that you have been applying your analytical
powers to the investigation of the acustical effects of corpuscular
forces, and that, among many more refined determinations, you, have
confirmed several of the results relating to sounding bodies, which
were published twenty years ago in my Lectures on Natural Philosophy:
though they were generally such as might have been derived from
the calculations of Bernoulli and Euler; which I attempted in some
[p449] measure to simplify by the introduction of the element which
I called the _Modulus of Elasticity_ of each substance. You have
very properly observed that it is often difficult to represent the
combination of these corpuscular forces by an integral, since in many
practical cases the integral must vanish, where it would naturally be
applied to the phenomena: and, from similar considerations, I trust
you will be prepared to admit the objections that I made long ago,
to the reasoning of your great predecessor, Mr. Laplace, to whose
station in the mathematical world you appear so eminently qualified
to succeed.
The equation, which may be called final, in Mr. Laplace’s Supplement
to the Xth Book, p. 47, is _Q_ cos. (ω−θ) = (2ς−ς′) _K_ sin. θ. Now
this, in my opinion, is a perfect _reductio ad absurdum_: for _Q_
must _always_ be _incomparably_ less than _K_; the attraction of
the particles lying between a cylinder and its tangent plane being
_always_ infinitely less than that of the particles in an angular or
prismatic edge: or if this were denied in general, it would obviously
become true when the cylinder itself becomes a plane, and _Q_
vanishes altogether; which will always be the state of the problem,
when the surface of the solid is so inclined to the horizon, that the
surface of the fluid may remain horizontal, the appropriate angle of
contact being unaltered in these circumstances, as it is easy to show
by making the experiment with mercury.
I entreat you to consider this objection with patient attention, and
to tell me if you can find any arguments to supersede it. I would
also presume to ask your opinion of my own method of deducing the
force of capillarity from the elementary attractions and repulsions
of bodies, at the end of my Illustrations of the Celestial Mechanics,
Art. 382; Appendix A, p. 329 to 337. The volume is in the Library
of the Academy; or I should have taken the liberty of sending you a
copy, as an inadequate return for so many valuable communications
with which you have had the kindness to favour me.
Believe me always, dear Sir,
Very truly yours,
* * * *
_London_, 18 _Nov._ 1827.
[p450]
iv. _Calculations of_ LUNAR PHENOMENA. _By_ THOMAS HENDERSON, Esq.
----------------------------------------------------------------------+
Principal LUNAR OCCULTATIONS of the Fixed Stars in the |
Months of January, February, March, and April, 1828; calculated |
for the Royal Observatory at Greenwich. |
---------+---------------+------+--------------+-------------+--------+
| | | Immersion and| Apparent |Point of|
Date. | Names of |Magni-| Emersion. |Difference of| Moon’s |
| Stars. |tude. | Mean Time. |Declination. | Limb. |
---------+---------------+------+--------------+-------------+--------+
| | | H. M. S. | ′ * ″ | ° |
Jan. 4|κ Cancri | 5.6 |Imm. 10 51 43 | 13 18 S. | 172 R. |
| | |Em. 11 44 48 | 7 19 S. | 91 R. |
31|α^1 Cancri | 6 |Imm. 11 14 52 | 7 45 S. | 134 L. |
| | |Em. 12 35 56 | 2 19 N. | 88 R. |
" |κ Cancri | 5.6 |Imm. 18 38 0 | 0 58 N. | 47 L. |
| | |Em. Under | Horizon. | |
Feb. 7|α^2 Libræ | 3 |Imm. 20 29 54 | 1 35 S. | 69 L. |
| | |Em. 21 37 54 | 4 33 N. | 107 R. |
22|δ^3 Tauri | 5 |Imm. 7 0 9 | 3 47 S. | 90 L. |
| | |Em. 8 16 39 | 6 34 S. | 146 R. |
28|ω Leonis | 6.7 |Imm. 11 24 25 | 14 57 S. | 165 L. |
| | |Em. 12 3 15 | 9 17 S. | 145 R. |
March 10|ρ^1 Sagittarii | 5 |Imm. 16 14 54 | 4 24 N. | 105 L. |
| | |Em. 17 20 39 | 1 25 N. | 62 R. |
23|u Geminorum | 5.6 |Imm. 8 4 36 | 2 48 N. | 55 L. |
| | |Em. 9 18 11 | 7 46 N. | 94 R. |
24|k Geminorum | 5 |Imm. 9 12 5 | 3 53 S. | 78 L. |
| | |Em. 10 28 34 | 3 55 N. | 111 R. |
26|κ Cancri | 5.6 |Imm. 7 41 25 | 7 6 S. | 132 L. |
| | |Em. 9 3 38 | 3 14 N. | 83 R. |
April 2|ν^1 Libræ | 6 |Imm. 14 7 43 | 12 43 N. | 38 L. |
| | |Em. 14 34 56 | 15 49 N. | 10 R. |
|ν^2 Libræ | 6.7 |Imm. 13 58 36 | 2 1 S. | 99 L. |
| | |Em. 15 13 58 | 6 19 N. | 76 R. |
29|α^1 Libræ | 6 |Imm. 16 15 38 | 14 48 S. | 126 L. |
| | |Em. 16 48 16 | 11 55 S. | 174 R. |
|α^2 Libræ | 3 |Imm. 16 33 5 | 15 54 S. | 145 L. |
| | |Em. 16 43 5 | 15 3 S. | 162 L. |
---------+---------------+------+--------------+-------------+--------+
The fifth column shows the apparent difference of declination
between the Star and Moon’s centre at the immersion and emersion;
the letters N and S denoting the Star to be north or south from the
Moon. The sixth or last column shows the point of the Moon’s limb
where the immersion and emersion take place, reckoning from the
vertex or highest point; the letters L and R signifying to the left
hand or right hand of the observer.
An error of 11 seconds in the computed difference of declination
between the Moon and Star, will be sufficient to convert the
expected Occultation of α^2 Libræ, on 29th April, into an Appulse;
and a less error will considerably affect the times and places of
immersion and emersion.
[To be continued.]
+----------------------------------------------------------------------+
[p451]
+----------------------------------------------------------------------+
ELEMENTS for computing the ECLIPSES of the SUN and OCCULTATIONS of
the PLANETS by the Moon, in the Year 1828.
Conjunction in Diff. Relat Relative ☉ or Pla ☉ or Pla
A. R. Apparent Dec. ive Orb. Ang. net’s A. net’s N.
Time. H. M. R. at ☌ P. D. at ☌
☽ D. H. M. S. ′☽ ″ ′ ″ ° ′ H. M. S. ° ′ ″
♃ Jan. 11 10 47 11 21 1 S. 34 6 S. 76 58 E. 14 35 47 104 2 30
♂ Jan. 11 16 40 36 4 29 N. 33 14 S. 78 4 14 49 29 105 12 34
♃ Feb. 7 22 17 54 5 44 N. 33 26 S. 77 35 14 46 40 104 48 7
♃ Mar. 6 4 48 7 16 46 N. 33 29 S. 77 47 14 49 7 104 53 57
♃ April 2 8 6 49 9 6 N. 34 12 S. 77 26 14 42 44 104 21 7
☉ April 13 21 23 53 8 50 N. 31 23 N. 74 7 1 30 20 80 32 14
♃ April 29 10 49 53 9 3 S. 34 50 S. 76 40 14 30 21 103 22 5
☿ May 12 8 58 41 7 44 N. 27 31 N. 78 28 2 30 50 76 35 6
♃ May 26 14 58 46 20 58 S. 34 38 S. 75 54 14 17 53 102 23 57
♃ June 22 21 27 33 14 27 S. 33 40 S. 75 26 14 11 5 101 55 51
♀ July 13 12 0 30 68 43 S. 30 15 S. 76 50 8 55 0 76 17 27
♃ July 20 6 18 33 10 12 N. 32 33 S. 75 28 14 12 35 102 11 28
♃ Aug. 16 17 18 6 44 49 N. 31 50 S. 75 59 14 22 10 103 7 21
♀ Sept. 5 3 7 12 4 10 S. 28 8 S. 78 3 8 13 4 75 7 21
☉ Oct. 8 12 23 35 6 39 S. 29 6 S. 73 2 12 57 44 96 10 27
♀ Dec. 3 13 30 46 39 10 S. 29 39 S. 75 23 14 3 58 100 19 38
Conjunction in A. Nearest Time of { ☉ or Planet’s }
R. Apparent Time. Approach. nearest Horary Motion Semi- Hor.
Approach, in A. in N. diam- Par.
Apparent R. in P. D. eter
Time. Time.
☽ D. H. M. S. ′ ″ D. H. M. S. SEC. ″ ″ ″
♃ Jan. 11 10 47 11 20 28 11 10 38 51 + 1·3 + 6 17 2
♂ Jan. 11 16 40 36 4 23 11 16 42 16 + 5·9 + 27 3 5
♃ Feb. 7 22 17 54 5 36 7 22 20 7 + 0·6 + 2 18 2
♃ Mar. 6 4 48 7 16 23 6 4 54 29 − 0·2 − 1 20 2
♃ April 2 8 6 49 8 53 2 8 10 18 − 0·9 − 4 21 2
☉ April 13 21 23 53 8 30 13 21 19 16 + 9·2 − 54 958 9
♃ April 29 10 49 53 8 48 29 10 46 17 − 1·2 − 6 22 2
☿ May 12 8 58 41 7 35 12 8 55 19 +19·8 −119 3 7
♃ May 26 14 58 46 20 20 26 14 49 55 − 0·9 − 4 21 2
♃ June 22 21 27 33 13 59 22 21 21 5 − 0·3 − 1 20 2
♀ July 13 12 0 30 66 55 13 11 29 28 − 3·4 + 22 26 27
♃ July 20 6 18 33 9 53 20 6 23 16 + 0·5 + 3 18 2
♃ Aug. 16 17 18 6 43 29 16 17 38 31 + 1·2 + 6 17 2
♀ Sept. 5 3 7 12 4 5 5 3 5 22 + 5·8 − 1 18 19
☉ Oct. 8 12 23 35 6 21 8 12 19 35 + 9·2 + 57 963 9
♀ Dec. 3 13 30 46 37 54 3 13 10 46 +11·5 + 61 7 8
The places of the Sun and Moon have been taken from the Nautical
Almanac, those of Mercury from Lindenau’s Tables, and those of the
other Planets from Schumacher’s Ephemeris.——The sign + denotes the
motion in A. R. to be direct; the sign −, retrograde. The sign +
denotes the motion in N. P. D. to be towards the South; the sign −
towards the North.——None of the preceding Conjunctions will prove
to be an Eclipse or Occultation visible at Greenwich.
+----------------------------------------------------------------------+
[p452]
+-----------------------------------------+
Apparent Distance of Jupiter’s Satellites
from Jupiter’s Centre, at his Conjunctions
in A. R. with the Moon.
+------------+----------+-----------------+
Date. Satellite. Distance.
+------------+----------+-----------------+
1828. ′ ″
+------------+----------+-----------------+
January 11 I. 1 14 East
II. 0 51 ----
III. 4 16 West
IV. 2 21 ----
February 7 I. 1 6 West
II. 2 40 East
III. 2 16 West
IV. 4 27 ----
March 6 I. 1 51 East
II. 2 24 West
III. 3 14 East
IV. 8 44 ----
April 2 I. 1 44 West
II. 0 15 ----[A]
III. 4 58 East
IV. 7 30 West
29 I. 0 14 West[B]
II. 2 51 East
III. 0 32 West
IV. 1 6 East
May 26 I. 1 44 East
II. 3 8 West
III. 5 15 ----
IV. 6 38 East
June 22 I. 1 59 West
II. 0 4 East[C]
III. 3 10 West
IV. 8 56 ----
July 20 I. 1 51 East
II. 2 50 ----
III. 1 55 ----
IV. 4 51 ----
August 16 I. 1 43 West
II. 1 12 ----
III. 4 21 East
IV. 1 53 ----
+------------+----------+-----------------+
A: On Jupiter’s disk. B: Eclipsed.
C: On Jupiters disk.
These Configurations have been computed
from De Lambre’s Tables.
+-----------------------------------------+
[p453]
MISCELLANEOUS INTELLIGENCE.
I. MECHANICAL SCIENCE.
1. _On the Adhesion of Screws_.—The following results, respecting the
force necessary to draw iron screws out of given depths of wood, are
by Mr. Bevan, and should be placed by the side of those he has given
with regard to nails[128].
“The screws I used were about two inches in length, 0.22 diameter at
the exterior of the threads, 0.15 diameter at the bottom, the depth
of the worm or thread being 0.035, and the number of threads in one
inch = 12. They were passed through pieces of wood exactly half an
inch in thickness, and drawn out by the weights specified in the
following table:
Dry beech 460 pounds
Do. Do. 790
Dry sound ash 790
Dry oak 760
Dry mahogany 770
Dry elm 655
Dry sycamore 830
“The weights were supported about two minutes before the screws were
extracted.
“I have also found the force required to draw similar screws out of
deal and the softer woods about half the above.
“From which we may infer as a rule to estimate the _full_ force
of adhesion, in hard wood . . . 200.000 _d_ δ _t_ = _f_,
and in soft wood . . . 100.000 _d_ δ _t_ = _f_,
_d_ being the diameter of the screw; δ the depth of the worm
or thread; and _t_ the thickness of the wood into which it is
forced;—all in inches; _f_ being the force in pounds to extract the
same.” We may, from the above experiments, observe the approximation
to perfection in the art of screw making; for had the screw been
greater in diameter, there would have been a waste of material, or
had it been less, it would not have been sufficiently strong, which
may be proved as follows: the cohesion of wrought iron has been
found, from a number of experiments, to be about 43000 lbs. per
cylindrical inch; and as the smallest diameter of screw used in my
experiment was 0.15, it would have been torn asunder by a force of
about 968 lbs.; or if the hard wood had been about 5/8 of an inch
thick into which it had been screwed, the screw would have been
broken instead of forcing its passage out of the wood.—_Phil. Mag. N.
S._ ii. 291.
FOOTNOTE:
[128] See page 360, vol. xvii. of the former series of this Journal.
2. _Improvement in Steam-engines_.—According to the valuable records
kept of the duty of the steam-engines at the mines in Cornwall,
a most important improvement has been effected in two [p454]
instances, of engines erected by Captain Samuel Grose; dependent
entirely upon attention to the smaller details of the machines. The
best engines, heretofore, had not done more than raise forty millions
of pounds of water one foot high, by each bushel of coals consumed,
except indeed upon short occasions. In one of the cases in question,
an engine at Wheal Hope, of sixty-inch cylinder, working single as
usual, the duty rose to fifty, fifty-four, and fifty-five millions of
pounds; and in the other, an engine of eighty-inch cylinder, at Wheal
Towan, the duty rose in
April 61,877,545
May 60,632,179
June 61,762,210
July 62,220,820
August 61,764,166
thus exceeding by nearly fifty per cent. what had been effected
before that time.
3. _Improved Clock_.—Among the articles displayed at the first
National Exhibition of the Objects of Arts and Industry, at
Neufchatel, Switzerland, last year, was a clock made by F. Houriet,
of Locle; in which steel was used only in the main springs and in the
axes of the moveable parts; all the other parts were in brass, gold
alloy, and white gold. The number of pieces in gold, gold and silver,
gold and platina, is sixty-two: all the pivots turn on jewels, and
the functions of the free escapements are effected also by means of
pallets in precious stones. It had been supposed that the escapements
and the spiral spring not being of steel, inconvenience would result
from the smaller degree of elasticity, but numerous trials with
favourable results have removed the objection; and it appears that
gold, hardened either by hammering or other means, is more elastic
than hardened and untempered steel. The clock had gone for six days,
exposed to the contact of a magnet competent to lift twenty-five or
thirty pounds, without suffering any derangement.—_Rév. Ency._
4. _Method of dividing Glass by Friction_.—The following method is
described by Dr. Hare: “Some years ago Mr. Lukin showed me that a
small phial or tube might be separated into two parts, if subjected
to cold water after being heated by the friction of a cord made to
circulate about it, by two persons alternately pulling in opposite
directions. I was subsequently enabled to employ this process in
dividing large vessels of four or five inches in diameter, and
likewise to render it in every case more easy and certain by means
of a piece of plank forked like a boot-jack, and also having a
kerf cut by a saw, parallel to and nearly equidistant from the
principal surfaces of the plank, and at right angles to the incisions
productive of the fork.
“By means of the fork, the glass is easily held steadily by the
hand of one operator; by means of the kerf, the string, while
[p455] circulating about the glass, is confined to the part where
the separation is desired. As soon as the cord smokes, the glass is
plunged in water, or if too large to be easily immersed, the water
must be thrown upon it; the latter method is always preferable when,
upon immersing the body, the water can reach the inner surface.
As plunging is the most effectual method of employing the water
in the case of a tube, I usually close the end which is to be
immersed.”—_Silliman’s Journal_, xiii. 7.
5. _Use of Soapstone in diminishing Friction_.—In a letter to
Professor Silliman upon this subject, Mr. E. Bailey of Boston, says,
“I understand the Soapstone has been used for this purpose in the
extensive manufactories at Lowell, for about two years, and with
great profit and success. Besides answering the purpose to which it
is applied very much better than any other substance that can be
procured, it saves a great deal of trouble and expense. It is first
thoroughly pulverized, and then mixed with oil, tallow, lard, or
tar, whichever may be the best adapted to the use for which it is
designed. It is of course important to procure that which is free
from _grit_, and it can be purified in a good degree by mixing the
powder with oil, and decanting it after it has stood a few minutes.
The heavier particles will form a sediment to be rejected. It is used
in all kinds of machinery where it is necessary to apply any unctuous
substance to diminish friction, and it is said to be an excellent
substitute for the usual composition applied to carriage-wheels.”
Some idea of the value of soapstone thus applied, may be formed from
the following fact communicated by D. Moody, Esq., the superintendent
of the tar-works on the mill-dam near this city. Connected with
the rolling machine of that establishment, there is a horizontal
balance-wheel, weighing _fourteen tons_, which runs on a step of
five inches diameter, and makes from seventy-five to one hundred
revolutions in a minute. About one hundred tons of iron are rolled in
this machine in a month; yet the wheel has sometimes been used from
three to five weeks without inconvenience, before the soapstone has
been renewed. The superintendent thinks, however, that it ought to be
more frequently employed.
“The use of soapstone was discovered at Lowell. It has been
said never to fail in producing the desired result when applied
to machinery which had began to be heated, even in those
cases when nothing else could be found that would answer the
purpose.”—_Silliman’s Journal_, xiii. 192.
6. _On peculiar Physical Repulsions, by_ M. Saigey.—I intend to give
in this bulletin the description of a very simple apparatus, by means
of which I have made many experiments, which have conducted me to the
following results:—
i. All bodies exert between themselves a feeble repulsive action in
ordinary circumstances. The repulsion between bismuth and [p456]
antimony and the poles of a magnetic needle, is a case of this
general law, and is not due to magnetism. Nor is it magnetism which
occasions the direction of needles formed of other substances than
iron, announced lately by M. Becquerel.
ii. A very marked attraction may be observed between a cold and a
heated body, or between two bodies of different temperature, whether
screens be interposed or not.
iii. The metallic plates in the Cabinet de Physique de Paris,
intended for the repetition of M. Arago’s experiments on magnetism by
rotation, contain more or less of iron capable of attracting a very
mobile magnetic needle. These plates, and those of M. Arago, were
made by the same person and from the same materials.
iv. I believe that, in many cases, results obtained without the
appreciable developement of magnetism or electricity, have been
attributed to these powers; and from well-proved experiments I shall
deduce new results relative to the diurnal variation of the needle,
the direction of the plumb-line and the density, temperature and
attraction of the planetary masses.—_Bull. Univ._ A. viii. 287.
7. _On the Magnetic Effects of Metals in Motion_.—M. Seebeck has
endeavoured to determine the effects of various metals in diminishing
the oscillations of a magnetic needle 2-1/8 inches in length, and
suspended by a silk fibre three lines distant from and above the
plates. The oscillations were counted from an amplitude of 45° to 10°.
116 oscillations above a plate of marble
112 layer of mercury 2 lines in thickness.
106 plate of bismuth 2 "
94 platina 0.4 "
90 antimony 2.0 "
89 lead 0.75 "
89 gold 0.2 "
71 zinc 0.5 "
68 tin 1.0 "
62 brass 2.0 "
62 copper 0.3 "
55 silver 0.3 "
6 iron 0.4 "
It is also stated that he has found, from experiments, that by
alloying such metals as are magnetic, like iron, nickel, and cobalt,
with other metals, which like antimony diminish the magnetic force,
alloys are obtained entirely neutral in their effects; thus the
alloys formed by four of antimony with one of iron, three of copper
with one of antimony, and two of copper with one of nickel, produce
no diminution of the number of oscillations, these amounting to 116
as with the plate of marble. These three alloys are, therefore, the
best for the manufacture of compasses, those of copper and nickel
being the most malleable.—_Annal. des Phy. 1826. Bull. Univ._ A.
viii. 136. [p457]
8. _Duration of the Effects of Light upon the Eye_.—M. Plateau of
Liege has endeavoured to determine the length of time during which
the impression of certain luminous rays upon the eyes remains; and
has given the following results:
Flame 0″.242
Ignited Charcoal 0″.229
White 0″.182
Blue 0″.186
Yellow 0″.173
Red 0″.184
9. _On the Measurement of the Intensity of Light, by_ M. Peclet.—A
very usual photometrical process is to interpose an opaque body
between a white screen and the two lights to be measured, and to move
the latter until the shadows produced are of equal intensity; the
intensity of the lights being then as the square of their distances
from the shadows they illuminate. Sometimes a translucent body, as
unpolished glass or oiled paper, is used in place of an opaque one,
the shades produced by transmission being observed.
In both these methods, the apparent intensity of the shadow varies
with the position of the observer. If the shadows are equal when
observed from a point perpendicular to the white screen at the middle
of the distance of the two shades, they will be no longer so on
removing from that position, and the shadow nearest to the observer
will always appear the darkest. These apparent variations are greater
as the shadows are farther apart, or with reflected shadows as the
screen is smoother, or with transmitted shadows as the interposed
obstacle is more diaphanous.
The explanation given of this fact is, that unpolished opaque bodies,
like paper, plaster, &c. never disperse the light incident upon them,
in an uniform manner, more rays passing in the direction in which
regular reflexion would take place, than in any other. Hence, when
two equal shadows are produced upon such a surface, either by two
equal lights at equal distances, or by two unequal lights at unequal
distances; the shadow nearest to the observer must necessarily appear
deeper than the other, because it is enlightened by the nearest
light, the rays from which are reflected in greatest abundance away
from the observer; and, on the contrary, the shadow further from the
observer should appear lightest, because the rays which fall on it
from the furthest light are reflected in greatest abundance towards
the side on which the observer stands. The reason, also, why the
effect is greater as the shadows are further apart is evident; and
why in every case it is reduced to nothing when the observer is in
a plane perpendicular to the screen and equidistant from the two
shadows.
From these facts and explanations it may be concluded, that, in all
photometrical measurements by reflected shadows, the screens should
have all smoothness removed from them, and the two [p458] shadows
brought as near together as possible, and even made to touch or
over-lap; or that, when this cannot be done, the observation should
be made from a point equidistant from the two shadows. As to the
shadows by transmission, the apparent variations of intensity are so
great for small changes in the position of the eye, as to render the
method altogether inapplicable.—_Bull. Univ._ A. viii. 248.
10. _On the apparent Decomposition of White Light by a Reflecting
Body when in Motion_.—The following experiment is described in the
MSS. of M. Benedict Prevost and published by M. P. Prevost. A ray
of solar light being introduced into a darkened chamber, is to have
a square piece of white paper about two inches in the side, passed
across it perpendicularly to the direction of the ray. The light
reflected by the paper, instead of being white, will present a small
white central portion, surrounded by the seven principal colours,
nearly in the order of the prismatic spectrum. When a red surface is
used instead of a white one, the decomposition of the light is still
more complete. When the paper has a slight blue tint, the effect
is less perfect than with the white paper. With a black surface no
colours appear, but a sort of smoky shade towards the middle. A
single passage of the paper is sufficient, but it is necessary that
it pass entirely through the ray, no part remaining in it.—_Bib.
Univ._—_Bul. Univ._ A. viii. 248.