164 Prof. D. B. Brace on a 



rotary effects. Other defects usually found are imperfect 

 polishing and figuring of the nicols themselves. These are 

 usually only observed in work with spectral colours from a 

 narrow source giving approximately a single bundle of rays. 

 ^ The scratches and ridges here become at once evident and 

 any variation in the curvature of the face manifests itself in 

 a variation of intensity of the field, producing a shaded or 

 mottled appearance. This is a well-known appearance * in 

 spectrophotometry when the ray system is not coaxial with 

 ' othe lenses or when their curvature is not uniform. 



In the nicol prism a total reflexion of the ordinary ray 

 takes place at the internal diagonal faces which are cemented 

 with balsam or with evaporated turpentine-oil or linseed-oil, 

 whose indices are less than that for the ordinary ray. This 

 allows a part when balsam, or practically all when these oils are 

 used, of the extraordinary ray to pass through. The reflected 

 ray is absorbed by the sides of the nicol but not completely. 

 When an intense source like the sun is used the diffused light 

 is so large as to produce a decided effect on the hue when 

 jv colours of very low intensity reach the eye. This of course 

 is a difficulty which the optician can eliminate by proper 

 casing and diaphragming. 



It is not a difficult matter mechanically and optically to 

 reverse the conditions of reflexion with the optical media now 

 2 < available. For example, we have as the index of the ordinary 

 ray of spar for the D-line 1'6584, while for the same line the 

 value for a-monobromonaphthaline is 1*6582. Schott and 

 Co., of Jena, also cast a glass the value of which is n D = 1*6527. 

 The dispersion of each of these is of course different, but for 

 the mean colours of the spectrum the indices are almost the 

 same. It has seemed heretofore impossible to obtain glass so 

 free from strain as to produce no observable depolarization. 

 The use of liquids, however, has been found perfectly practi- 

 cable and efficient. Solutions of carbon disulphicle may be 

 used giving almost any convenient index from a low value 

 with balsam to an index greater than 1*7 D with phosphorus. 



The well-known optical fluid a-monobromonaphthaline is, 

 however, much superior, as it remains nearly colourless if 

 sealed up aud not subjected to actinic action by continuous 

 exposure to light. Its index is the same as that of spar for 

 the mean portions of the spectrum. If now the cementing 

 film and the prisms of spar are interchanged, we should have 

 an interchange of the extraordinary and ordinary rays. Thus 

 a plate of spar placed diagonally at the proper angle within a 

 * Tuckerman, Astrophys. Journ. xvi. p. 145. 



