After describing other methods of determining the sensitiveness of
plates, Mr. G.F. Williams, in the _Br. Jour. of Photo_., thus explains
his plan. I will now explain the method I adopt to ascertain the
relative sensitiveness of plates to daylight. Procure a small direct
vision pocket spectroscope, having adjustable slit and sliding focus. To
the front of any ordinary camera that will extend to sixteen or eighteen
inches, fit a temporary front of soft pine half an inch thick, and in
the center of this bore neatly with a center bit a hole of such diameter
as will take the eye end of the spectroscope; unscrew the eyehole, and
push the tube into the hole in wood, bushing the hole, if necessary,
with a strip of black velvet glued in to make a tight fit. By fixing the
smaller tube in the front of camera we can focus by sliding the outer
tube thereon; if we fix the larger tube in the front, we should have to
focus inside the camera, obviously most inconvenient in practice. Place
the front carrying the spectroscope _in situ_ in the camera, and rack
the latter out to its full extent; point the camera toward a bright sky,
or the sun itself, if you can, while you endeavor to get a good focus.
The spectrum will be seen on the ground glass, probably equal in
dimensions to that of a quarter plate. Proceed to focus by sliding the
outer tube to and fro until the colors are quite clear and distinct, and
at same time screw down the slit until the Fraunhofer lines appear. By
using the direct rays of the sun, and focusing carefully, and adjusting
the slit to the correct width, the lines can be got fairly sharply.
Slide your front so that the spectrum falls on the ground glass in just
such a position as a quarter plate glass would occupy when in the dark
slide, and arrange matters so that the red comes to your left, and the
violet to the right, and invariably adopt that plan. It is advisable to
include the double H lines in the violet on the right hand edge of your
plate. They afford an unerring point from which you can calculate
backward, finding G, F, E, etc., by their relative positions to the
violet lines. Otherwise you may be mistaken as to what portion of the
spectrum you are really photographing. The red should just be seen along
the left edge of the quarter plate. When all is arranged thus, you
utilize three-fourths of your plate with the spectrum, with just a
little clear glass at each end. Before disturbing the arrangement of
the apparatus, it is desirable to scratch a mark on the sliding tube,
and make a memorandum of the position of all the parts, so that they may
be taken away and replaced exactly and thus save time in future.
To take a photograph of the spectrum, put a quarter plate in the dark
slide and place in camera; point the camera toward a bright sky, or
white cloud, near the sun--not at the sun, as there is considerable
difficulty in keeping the direct rays exactly in the axis of the
spectroscope--draw the shutter, and give, say, sixty seconds. On
development, you will probably obtain a good spectrum at the first
trial. The duration of exposure must, of course, depend upon the
brightness of the day; but if the experiments are to have relative
values, the period of exposure must be distinctly noted, and comparisons
made for a normal exposure of sixty seconds, ninety seconds, two minutes
or more, just according to whatever object one has in view in making the
experiments. With a given exposure the results will vary with the light
and the width of the slit, as well as being influenced by the character
of the instrument itself. Further, all such experiments should be made
with a normal developer, and development continued for a definite time.
The only exception to this rule would be in the event of wishing to
ascertain the utmost that could be got out of a plate, but, under
ordinary circumstances, the developer ought never to vary, nor yet the
duration of development. To try the effect of various developers, or
varying time in development, a departure must be made of such a nature
as would operate to bring out upon each plate, or piece of a plate, the
utmost it would develop short of fog, against which caution must be
adopted in all spectrum experiments.
On development, say for one, two, or three minutes, wash off and fix.
You will recognize the H violet lines and the others to the left, and
this experiment shows what is the sensitiveness of this particular plate
to the various regions of the spectrum with this particular apparatus,
and with a normal exposure and development. So far, this teaches very
little; it merely indicates that this particular plate is sensitive or
insensitive to certain rays of colored light. To make this teaching of
any value, we must institute comparisons. Accordingly, instead of simply
exposing one plate, suppose we cut a strip from two, three, four, or
even half a dozen different plates, and arrange them side by side,
horizontally, in the dark slide, so that the spectrum falls upon the
whole when they are placed in the camera and exposed. There is really no
difficulty in cutting strips a quarter of an inch wide, the lengthway of
a quarter plate. Lay the gelatine plate film up, and hold a straight
edge on it firmly, so that when we use a suitable diamond we can plow
through the film and cut a strip which will break off easily between the
thumb and finger. A quarter plate can thus be cut up into strips to
yield about a dozen comparative experiments. When cut and snapped off,
mark each with pencil with such a distinguishing mark as shall be
clearly seen after fixing. The cut up strips can be kept in the maker's
plate box.
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The deep down underground electric railway in London has so far proved
an unprofitable concern for its stockholders. It is 3½ miles long,
touches some of the greatest points of traffic, but somehow or other
people won't patronize it. The total receipts for the last six months
were a little under $100,000, and they only carried seventeen persons
per train mile. On this road the passengers are carried on elevators up
and down from the street level to the cars. The poor results so far make
the stockholders sick of the project of extending the road.
* * * * *