348 Proceedings of Royal Society of Edinburgh. [sess. 
It may, however, be noted that the chief objection to a some- 
what long telescope tube can be done away with by the use in the 
spectrophotometer of a “constant deviation prism, ”* a construc- 
tion of prism which permits the telescope of the instrument to be 
permanently fixed, while the prism alone rotates to bring the 
different parts of the spectrum to the observer’s eye. In the case 
of a spectrophotometer furnished with such a prism the rigid 
mounting of even an unusually long telescope tube of course 
presents no difficulty. 
Either of the above described arrangements of spectrophotometer 
having been adopted, it might be supposed that the similar strips 
of the two adjacent spectra could be satisfactorily observed on 
looking at them through any ordinary eyepiece. What is thus 
Fig. 7. 
seen, however, is unsatisfactory. The two luminous strips are not 
like natural objects, which give out rays of light in all directions 
from every point, but on the contrary the edge of each of the 
strips brought into contact gives out rays of light only in a single 
plane, as indicated in fig. 8. From any point a of the upper edge 
of the lower image rays proceed only in the plane normal to the 
paper which passes through the line aB, and similarly from any 
point b of the lower edge of the upper image the rays proceed only 
in the plane normal to the paper which passes through the line b A. 
Accordingly, the coincident edges of the two images are seen by 
means of two sets of rays which respectively fall on the optical 
system of the eye at places some distance apart. Now through 
the effects of the eye’s spherical aberration, and probably also 
because of general irregularities in the refractive parts of the eye, 
the two sets of rays from the coincident edges of the strips will 
not be brought to the same line on the retina. Any slight move- 
* As employed, for example, by Messrs Hilger, Ltd., on certain of their 
spectrometers. 
