362 Notices respecting New Books. 



3. If the direction of the wave-normal within the prism 

 does not quite coincide with the axis of rotation, the average 

 error in the position of the plane of polarization is less than 

 for any other method of cutting. 



I hope shortly to have some prisms cut by Mr. Hilger in 

 this manner, and to test by means of them the theoretical 

 conclusions arrived at in the paper. 



Note added April 26th. 



If the plane of section PQ be inclined to BC at an angle 

 of 20°, as in fig. 2, the angular aperture of the field will be 

 small, only about 10°, and it will be necessary that all the 

 light traversing the prism should be very nearly parallel to 

 BC. The aperture may be increased up to about 20° by 

 lengthening the prism considerably and decreasing the angle 

 between PQ and BC. If this be reduced to 11°, the aperture 

 will have its maximum value of 22°. 



The aperture may be somewhat increased, and the length 

 of the prism shortened, by using as the separating medium 

 balsam of copaiba, as was suggested at the meeting of the 

 Physical Society at which this paper was read. 



The mean index of refraction for this substance is about 

 1*52, as determined by Brewster. The angle of total reflexion 

 therefore for the ordinary ray is sin -1 (1*52/ 1*66), or about 

 66° , while for Canada balsam this angle is about 68°. The 

 possible aperture, using the balsam of copaiba, thus is about 

 24°. 



Professor Thomson's prism, mentioned already, will have a 

 wider field. But it must be remembered that the new prism 

 was not designed for microscopic work, but to obviate the 

 displacement in the image referred to at the commencement 

 of the paper, and to produce a field in which the plane-polari- 

 zation should be as nearly as possible complete. 



LIL Notices respecting New Books. 



Physical Optics (Text-boohs of Science). By E. T. G-lazebkook, 

 M.A. fyc. (London: Longmans, Green & Co. Pp. 434+xiv.) 



THIS is an important elementary work, chiefly on "physical optics f 

 but it contains also much of what is commonly termed " geome- 

 trical optics." It is on the whole most lucidly written, and gives 

 a capital idea of the subject to those who wish to grasp a sound 

 knowledge of it without going into the higher mathematical 

 analysis. The work purports to be adapted for " artisans and 

 students in public and science schools : " it is, however, partly from 

 the general difficulty of parts of the subject and partly from the 



