MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
017 
slit in each case being placed at the focus of the achromatic lens combination of nine 
inches fecal length and two inches aperture. The lens combination of each consists 
of two lenses, one hard crown and the other dense hint, both perfectly colourless. 
The lenses are not cemented together by balsam or other mounting medium, but are 
held in metal mounts and slightly separated from each other by a brass ring, so that 
they include between them an air space. By adopting this arrangement there is no 
risk of the setting being disturbed by the long-continued passage of the heat rays 
from a powerful source of light; and there is consequently no necessity for the 
troublesome interposition of a cell containing alum or any other liquid for the purpose 
of hltering out the heat rays. The spherical aberration was approximately corrected 
by making the outside surface of the crown-glass lens to deviate slightly from the 
spherical figure, and the final corrections for both spherical and chromatic aberration 
were effected by adjustment of the amount of separation of the two lenses. The 
1, 
comparatively large aperture of two inches, together with the short focal length of 
nine inches, allows of the passage of a large amount of light while rendering the 
instrument compact. 
The slit of each optical tube is carried at the end of an inner tube, capable of the 
necessary amount of motion in and out of the wider tube which carries the lens 
combination, by means of a rack and pinion worked by a milled head. The slit is 
specially adapted for the purpose in view by being constructed so that the two jaws 
move equally in opposite directions on each side of the central line of contact. This 
is essential in order that, for different amounts of opening to suit crystal plates or 
prisms of different degrees of transparency or sources of light of different intensities, 
