1903 - 4 .] Mr Milne on a New Form of Spectrophotometer. 351 
can be seen by the observer. It will then be noticed that this 
strip has superimposed upon it another bright strip (C or D, fig. 10), 
at the end which has just been brought into view by this move- 
ment of the eyepiece. Hence it appears that really there are in 
all four bright strips, disposed as shown in fig. 10, the two 
with which we are concerned being the middle pair with their 
edges in contact along the line AB. How the two additional 
bright strips C and D are formed by KL and the other rays 
whose refraction is analogous, and it will be seen that the state- 
ment made above — that the rays so refracted may for our purpose 
be ignored — is justified. 
But there is a further advantage to be gained by such a use of 
a Wollaston prism. It will be remembered (see p. 343) that the 
use of a Yieroidt double slit in connection with this instrument 
Fig. 10. — Under normal conditions only the middle portion of the above can 
be seen through the eyepiece, C and D lying outside the field of view. 
to regulate and measure the light intensities of the two beams was 
found to be unsatisfactory owing to the great loss of light which it 
entailed, while a modified arrangement of the same kind had 
also to be discarded. How, with the arrangement of apparatus 
described above, by the mere addition of a Hicol prism to the 
eyepiece there is provided the necessary appliance for regulating 
the intensities of the two strips of light seen by the observer until 
a perfect match is attained. The rays r and s (fig. 4), after trans- 
mission through the Wollaston prism, pass out along the same 
straight line t , but remain distinct in this, that they are polarised 
in planes at right angles to each other. Accordingly, because of 
the Hicol in the eyepiece, rotation of the latter about its axis 
causes every possible variation from zero to infinity of the ratio of 
the intensities of the two strips of light seen by the observer. By 
means of a circular vernier or other device the position of the 
