350 Proceedings of Royal Society of Edinburgh. [sess. 
polarised, and polarised in the proper planes respectively, then in 
addition to the two coincident exit rays there will he two other 
non-coincident exit rays, making four exit rays in all, hut the 
divergent rays have no connection with our present purpose, and 
may be disregarded, as will be shown later. Suppose now that 
a suitable Wollaston prism be placed in the plane FF', fig. 4, 
then all the rays which go to form the edges of the two spectra in 
the plane FF' (two of which rays are indicated by C b and Da, 
fig. 8) proceed, after passing through the Wollaston prism, in one 
and the same horizontal plane through the eyepiece E. In this 
way all the rays from any point common to the coincident edges 
of the two spectra fall on the cornea of the observer’s eye in one 
f 
i 
Fig. 9. 
and the same straight line, so that the optical defects of the eye 
spoken of before do not cause any difficulties. 
As mentioned above, each ray incident on the Wollaston prism 
gives rise to two emergent rays. Considering then, for example, 
the point b (fig. 8), we see that the ray proceeding from it in the 
plane of the paper will, after passing through the prism, give rise 
to two emergent rays SH and KL (fig. 9). The ray KL will not 
be seen at all by the observer unless the angle COD be small and 
the power of the eyepiece low. In the model that the author has 
had constructed the distance CO is about 6 ‘5 inches and CD is 
equal to -6 inch, while the eyepiece is one of moderate power, and 
such rays as KL can only be seen by moving the eyepiece either 
up or down until the junction of the two bright strips has passed 
out of the field of view, so that only one of the two bright strips 
