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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 give