axis of Li is a plain grating or repUca G, and the diffracted
beam is received by the telescope lens L2, which is fitted to
the front of another, shorter, rectangular box, which can slide
inwards or outwards within a slightly larger rectangular box,
which opens outwards at PQ, so that at MN the whole spec-
trum is included. If X is the centre of MN, and Lj the
centre of the lens, the line L2X must make an angle with the
axis of the collimator = 9, where b sin 6 is the wave-length
of the central ray of light in the spectrum MN. The grating

6



82 THE SPECTROSCOPE

G must be fixed at the point where the Hne XL2 produced
meets the axis of the collimator. . The spectrograph is seen
in elevation in Fig. 46, B, where the base-board, to which the
collimator, grating, and camera are, attached, is .fixed by two-
stout wooden uprights, E and F^ to a heavy wooden block,
W. The camera hack is arranged with grooves to take either
a focussing screen or a dark-slide ; the spectrum appears on
the plate at MN, a long rectangular opening in the back per-
mitting it to act on the plate. By shifting the dark-slide
further down the grooved back between each exposure
several photographs may be taken on the one plate.
Such a spectrograph can be made at a very small cost with
the help of a carpenter, and for the convenience of the reader
the following dimensions are given of one in actual use by
the author, which gives a 5-inch spectrum between X = 3500'
and X = 7500:

Focal length of collimating lens 10 cm.
Diameter „ „ yz cm.
