122 



XX. — Addendum to the Paper ■ On the Application of Polarized 

 Light in Microscopic Investigations.'' By M. S. Legg, Esq. 



(Read December 22, 1847). 



In the last part of the Society's Transactions, at p. 84, is given a 

 description of the Nicol's prism, in which it is implied that the complete 

 separation of the two images is effected by means of glass employed 

 in its construction, and as this (as well as the explanation of the fig. 

 3, PI. 12) would convey an erroneous impression, I am desirous of 

 adding a more ample description, and at the same time of acknow- 

 ledging my obligations to the late Mr. Sanderson, of Edinburgh, for 

 information, and to Mr. Darker, of Lambeth, for his kindness in point- 

 ing out to me the details in the construction of the prism. 



* This instrument is described by the inventor in Jameson's Edin. 

 Phil. Journal (vol. vi. p. 83), and consists of a rhomb of Iceland spar, 

 in the proportion of 1*4 inch in length, and about '48 inch in breadth 

 and thickness, divided into two equal portions (see fig. 3, PI. 12) in a 

 plane passing through the acute lateral angles, and nearly touching 

 the obtuse solid angles. The sectional plane of each of these halves 

 must be carefully polished, and the portions cemented firmly with 

 Canada balsam, so as to form a rhomb similar to what it was before 

 its division : by this arrangement the ordinary and extraordinary rays 

 are so separated that only one of them is transmitted ; the cause of 

 this great divergence of the rays is considered to be owing to the ac- 

 tion of the Canada balsam, the refractive index of which (T549) is 

 that between the ordinary (1*6543) and the extraordinary (1'4833) 

 refraction of calcareous spar, and which will change the direction of 

 both rays in an opposite manner before they enter the posterior half 

 of the combination." 



In finishing these prisms, it is usual to cement a piece of thin glass 

 to that face of the prism which is most frequently exposed to dust, to 

 protect the spar from scratches ; but some double-image prisms, par- 

 ticularly those of moderately dividing power (called Biot's rhombs), 

 are composed of one half glass and the other half spar. 



