1903 - 4 .] Mr Milne on a New Form of Spectrophotometer. 349 
ment of the head will alter the paths of the two sets of rays 
through the optical system of the eye, and the effect of such a 
movement will he to cause an apparent relative motion, as seen by 
the observer of the really coincident edges of the two spectra. 
As a matter of fact the edges of the two spectra are seen by an 
observer to be slightly overlapping each other at one moment, 
while a moment later a slight gap will have made its appearance 
between them. This, no doubt, is due to movements of the head 
or eye. 
The author at first sought to remedy this defect by giving to the 
divided lens a focal length of about half a metre, which caused a 
reduction of the angle AaB of fig. 8, and a consequent reduction 
in the distance between the two sets of rays aB, bA when 
Fig. 8. 
entering the eye. A specially short eyepiece also was used, so 
that the eye of the observer might come as near the diverging 
point a as possible. These alterations, while undoubtedly effecting 
much improvement, were after all only palliative in their effect, 
and the comparatively great focal length of the divided lens 
necessitated a somewhat unwieldy length of telescope tube, a point 
that has already been dwelt upon. 
After various other methods had been considered without 
success the following means of overcoming the difficulty was 
finally discovered. Advantage was taken of the well-known fact 
that if a ray of light fall normally upon one of the faces of a 
Wollaston double image prism there proceeds from the other face 
two divergent rays which are polarised in planes at right angles to 
each other. If now — reversely — there fall on one of the faces of 
the Wollaston prism two converging rays of light inclined at the 
proper angle, these two rays will emerge from the opposite face of 
the prism in one and the same straight line normally to the face. 
It is true, of course, that unless the entering rays be each 
>tB 
