676 SUMMAKY OF OUREENT RESEARCHES RELATING TO 



In the text-books it is usual to tell students tliat in the Nicol 

 prism the ordinary ray is suppressed by total reflection, because the 

 ordinary index of refraction is greater than that of balsam, and that 

 the extraordinary ray is transmitted because the extraordinary index 

 of refraction is less than that of balsam. Neither of these statements 

 is completely true. All that its inventor claimed for the Nicol prism, 

 and all that it actually performs, is as follows : — The critical angle of 

 total reflection being diiferent for ordinary and extraordinary rays, 

 the ordinary ray is totally reflected and thrown out of the field at an 

 incidence at which the extraordinary ray is still transmitted, the 

 available field of polarized light being the region between the points 

 where the extraordinary ray itself vanishes by total reflection and the 

 ordinary ray enters by lack of total reflection. The former limit is 

 in all ordinary Nicol prisms marked by a broad blue iris or band of 

 colour, the latter is delimited by a curved band at the opposite side 

 of the field, in which, amidst a prevailing line of red and orange, a 

 system of interference-bands can be seen. The existence of these 

 interference-fringes was examined by the author in 1877, in a paper 

 which appeared in the ' Proceedings of the Physical Society of 

 London,' vol. ii. p. 157. In the Foucault prism a similar limi- 

 tation of the field occurs, interference-fringes being visible at both 

 limits. 



The refractive index of balsam for light of mean refrangibility 

 may be taken at 1*54, that of the ordinary ray in calc-spar as 1-66, 

 that of the extraordinary ray as 1 • 487. The reciprocals of these are 

 very nearly in the resjiective proportions of 65, 67, 60. The extra- 

 ordinary index, however, is 1*487 only for rays at right angles to the 

 crystallographic axis, having there a minimum, and increasing up to 

 1*66 for rays whose direction coincides with that of the axis. The 

 ellipsoidal wave-surface of the sheet of extraordinary waves lies partly 

 without and partly within the spherical wave-surface for Canada 

 balsam, while the spherical wave-surface of the sheet of ordinary 

 waves lies wholly within. Hence total reflection may occur for the 

 extraordinary as well as the ordinary rays, but of the extraordinary 

 rays only those can suffer total reflection which are situated in such a 

 direction with respect to the optic axis that their corresponding 

 portion of the ellipsoidal wave-surface lies within the spherical wave- 

 surface for balsam. As the Nicol prism is usually constructed, this 

 limit of possible extraordinary total reflection occurs for rays (in a 

 principal plane of section) inclined at about 1 0° to the balsam film, 

 giving rise to the limit of the polarized field marked off by the blue 

 iris before mentioned. 



Prof. Thompson has succeeded in widening the available field of 

 polarized light by constructing polarizing prisms in which this blue 

 iris, and the total reflection of the extraordinary ray which produces 

 it, are got rid of. This can be done by cutting the crystal so that (1) 

 the balsam film lies in a principal plane of section, and (2) the 

 crystallographic axis is at right angles to the axis of the prism. 



The result of this mode of orientation of the axis and film is to 

 gain 9° of angular aperture at this side of the " field," supposing the 



