THE BINOCULAR MICROSCOPE. 



125 



prism, to the upper inclines of which they are cemented with Canada 

 balsam. 



The angle of inclination to be given to the prisms must depend 

 upon the dispersive power of the flint and crown-glass employed. In 

 the combination that I have worked out, I have used, for the sake of 

 simplicity, some flint and crown that Mr. Smith kindly furnished me 

 with, in which the dispersive powers are exactly as two to one ; conse- 

 quently I have had to make the angle of the crown just double that of 

 the flint, in order to obtain perfect achromatism. The refractive power 

 of each must also be known, that we may determine the angles of the 

 prisms suitable for refracting the rays from the object-glass into the 

 two eyes, at a distance of nine inches, c, fig. 89, represents a ray of 

 light incident at right angles upon the under-surface of the flint-prism. 



On leaving the second surface, and entering the crown-prism, it is 

 slightly bent inwards ; and on finally emerging, it is refracted outwards 

 in the direction required. On looking through this prism, I could not 

 discover the slightest colour or distortion — it is almost like looking 



through a piece of plain glass ; and the loss of 



light ia so inappreciable, that it is difiicult to 



distinguish any difierence between an object 



and its refracted image. The base of the 



compound prism should not be larger than 



is sufl5cient to cover the stop of the lowest 



object-glass, in order that they may be made 



very thin. 



The method of applying the prism to 



the binocular microscope is shown by fig. 



90 : a a is the object-glass, b the prism placed 



as close behind it as the fittings will admit. 



The prism is set in an aperture in a flat disc 



of brass, which has a horizontal play in every 



direction, in order that it may be adjusted 



and fixed in such a position that the junc- 

 tion of the prisms may bisect the rays from 



the object-glass, and at the same time be at 



right angles to the transverse centres of the 



eye-piece ; c c are the two bodies of the mi- 

 croscope, provided with draw-tubes and the 



usual eye-pieces dd. The distance between 



them should be rather less than the average distance asunder of the 



eyes ; and in cases where these are very wide apart, we can pull 



fig. 90. 



