110 THE ROYAL SOCIETY OF CANADA 
collimating” plan of 0. Von Littrow. The slit is placed at (or near) 
the focus of the lens, and the gratingis placed on the other side of the 
lens in such a manner that the light returning through the lens is 
focussed just below the slit, as illustrated in Fig. 1, which represents 
a vertical section through the middle of the spectrograph and mount- 
ings. Sis the slit, L the lens placed about 23 ft. from the slit and G 
is the grating placed just behind the lens L and adjusted so that its 
ruled lines are parallel to the slit. The beam of light to be examined 
passes through $ spreading out to fill L, which renders it parallel before 
it reaches G, which is tilted to direct light back through the lens so that 
the latter focusses it below the slit. Here it may be examined with 
an eye-piece or photographed in the plate-holder C. The forward 
tilt of the grating is given by the screw J in the grating cell and suitable 
springs pressing against the back of the grating. The grating in its 
cell rests on the stand G’, which revolves in a cylindrical socket in the 
bottom of the cast iron end, B, of the spectrograph, and, by turning a 
handle H’ attached to a worm working in a toothed sector, K, which 
is clamped rigidly to the spindle G’, the grating may be rotated about 
an axis passing through its surface, thus diffracting any desired part 
of the spectrum of any order through the lens to C. By means of the 
vernier-pointer, V, readings to tenths of a degree may be made on the 
graduated arc, E, and a record of these readings with the corresponding 
wave-lengths is kept, so that by turning H’, any desired wave-lengths 
may be at once directed to the centre of C, and the grating is then 
clamped in this position. The lens may be shifted and clamped to any 
focus by means of the handle H, and the position of the pointer, F, is 
read on a millimetre scale attached to the bottom of B. 
The plate-holder, made to take 2-5 in. X 12 in. plates is clamped 
in the frame C’, which may be raised or lowered by rack and pinion, so 
that several strips of spectra may be put side by side on the same plate 
and spaced as desired by reference to a millimetre scale. 
In addition to these motions of the parts, the spectrograph, as a 
whole, may be rotated about its axis ' thus facilitating the study of the 
rotation of the sun by enabling the observer to reflect (by means of a 
suitable system of reflecting prisms such as described later) the limbs 
of the sun at opposite ends of any diameter, so that they are always 
tangential to the slit. In Fig. 1, A and B are the ends resting on the 
bearings A’ and B’, which are supported by the cement piers P, P’, built 
on the cement floor. The end, A, is of half-inch cast brass. It has a V- 
groove running around its circular rim into which the semi-circular 
cast-iron support A’ is bevelled to fit. The back of A is a rectangular 
1 Plaskett, Report of Chief Astronomer, 1907, p. 58. 
