ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 429 



ably make it the best of known polarizing instruments, whether 

 employed as an analyser or as a polarizer. Being constructed of a 

 perfectly colourless substance, it transmits the light without altering 

 the colour or dispersing the rays, and also without sensible diminution 

 by the two partial reflections at the surfaces of entrance and exit. 



A careful investigation of the course of the rays in this appa- 

 ratus shows, however, some rather considerable defects, arising 

 primarily from the direction in which the crystal is ordinarily cut, 

 and also from the nature of the medium hitherto employed to reunite 

 the two parts. 



As is well known, the Nicol prism is simply a parallelopiped of 

 Iceland spar, the length of which is equal to 3 • 7 times its thickness 

 (fig. 79), and which is cut in two along the diagonal A B which joins 

 the obtuse angles. The planes of section are carefully polished, and 

 then cemented together with Canada balsam, the index of refraction of 

 which (1'549) is intermediate between the ordinary index of the spar 

 (1*658) and the minimum of its extraordinary index (1-483). 



The limiting angle for the ordinary ray on the film of Canada 

 balsam being 69° 5', every ordinary refracted ray which is incident at 

 a more oblique angle undergoes total reflection. 



If, for instance, the ray o d enters obliquely at the face A C, it 

 will undergo at d a refraction which causes it to take the direction 

 df. If it forms with the plane A B an angle of 20° 5', this ray will 

 limit the field from which ordinary rays are excluded, since all such 

 rays arriving on the film of balsam at a greater angle would undergo 

 total reflection. Thus all the rays comprised between the extreme 

 directions o d and e A, ordinarily refracted in the spar, will be reflected 

 at/, and will form a luminous cone Jifg, which will be lost on the 

 blackened face C B. The extraordinary rays, on the contrary, their 

 index being lower than that of balsam, will traverse the latter, and 

 will spread out at their exit into the space i h. It is not, however, 

 the plane of the section which limits the field on the side A e. The 

 extraordinary ray, in proportion as it approaches this plane, makes, 

 with the principal axis of the spar, larger and larger angles ; its 

 index diminishes, it is true, but never reaches a value so small as to 

 traverse under all angles of incidence the film of balsam. Under 

 sufficiently large angles it undergoes in its turn total reflection. This 

 is the other limit of the field of the prism. The inequality of the 

 dispersive power of the balsam and of the spar reduces still more this 

 limit, and renders the field still smaller. 



As the index of the Canada balsam is very little inferior to that of the 

 spar for the ordinary ray, and the limiting angle for this ray is 69° 5', 

 it is necessary to make the prism of considerable length — equal, as 

 mentioned above, to 3 • 7 times the small side. The total length of 

 the prism is represented by the projection of the long diagonal on the 

 axis of the prism, i. e. four times the length of the small side. 



To obviate these inconveniences it has been proposed to employ 

 different cementing substances, and particularly balsam of copaiba, 

 the index of refraction of which is lower, which would allow of 

 the prism being shortened. But the prism was still always cut 



