534 



Transactions of the Society. 



Fig. 88. 



may be practically disregarded altogether ; an eye, even when 

 placed almost close to the prism, receiving only the direct beam. 

 This beam is, of course, perfectly polarized in one plane, and can 

 by a proper arrangement of the glass compensator be rendered 

 practically free from distortion and colour. 



Other methods of effecting the compensation have suggested 

 themselves in the course of my work, and I have obtained the 

 best results by adopting the arrangement represented in fig. 88. 

 In this, the glass compensating prism, instead of being 

 mounted separately, is cemented upon one of the 

 terminal faces of the compound spar-prism ; the angle 

 of this latter, and also of the other terminal face, being 

 suitably modified. 



This seems distinctly preferable to the original 

 arrangement, for several reasons. 



1, The total length of the compound prism is 

 rather less, being scarcely more than twice its breadth. 



2. The field is rather larger, so that the prism can 

 be used over deeper Microscope eye-pieces (A and B) 

 without any of the field of view being cut off. 



3. The whole arrangement is more compact, all the compo- 

 nents being firmly cemented together, and therefore not liable to 

 accidental displacement. 



4. There is less loss of light by reflection, the reflecting surfaces 

 being reduced to two. 



A ray of light entering the prism in a direction parallel to its 

 axis is divided into two rays ; one of which, on emergence, follows 

 a course parallel to that of the original incident ray, and is practi- 

 cally free from distortion and colour : the other ray is deviated to 

 the extent of about 59° 30' (for yellow sodium light), being, of 

 course, strongly coloured and distorted. The angular separation 

 is so great that this latter ray does not interfere with ordinary 

 observations. 



I hope that the prism, which has cost me much time and labour, 

 will meet with the approval of the Society, and take a place as a 

 useful accessory to the Microscope and other optical instruments. 



