1903-4.] Mr Milne on a New Form of Spectrophotometer. 347 
two spectra, but would not alter the positions of those edges. 
Each spectrum, however, may be tilted to the required amount 
by slightly rotating the corresponding lens-half about the general 
optical axis of the instrument. In order to preserve symmetry 
the respective rotations in opposite directions of the lens-halves 
should be to equal amounts. Even with so-called achromatic 
lenses this device will probably be found necessary. 
The limitation of the field of view seen by the eye to a similar 
narrow strip from each spectrum is obtained in this form of the 
instrument by an appropriate screen in the eyepiece. 
Finally, it may be noted that it is desirable, in the interests of 
good definition, to use spherical lenses in preference to cylindrical, 
and to avoid curvatures of too small radius. Accordingly, instead 
of the theoretical lens discussed above, it is better to substitute 
one having the curvature on one of its faces spherical, and having 
the other face a convergent cylindrical lens whose axis of figure 
is horizontal. The proof that such a lens can be equivalent to 
the former is part of the general theory of optics, and neither this 
proof nor any details as to the necessary focal lengths of the 
curvatures, etc., need be entered upon here. 
The author’s experiments with this form of the instrument have 
not been numerous, because he found that the cheap divided lens 
used by him in the model gave less satisfactory definition than the 
spherical divided lens employed in the model of the instrument 
first described. He believes, however, that with a well-made lens 
this second arrangement of instrument might perhaps be better 
than the other. 
