122 THE MICROSCOPE. 



powers above this a special arrangement is needed for 

 the prism, which must be set close behind the lens of 

 the -|th or y^th inch, in order to obtain an entire field 

 of view in each eye. 



A strong light should be avoided for the illumina- 

 tion of objects observed with the binocular microscope, 

 as direct rays tend to destroy the stereoscopic effect. 

 The illuminator that has been found to give an excel- 

 lent effect consists of three plano-convex lenses, so 

 combined as to give a very large area of light, as well 

 as great intensity. 



The improvement effected in Nachet's binocular eye- 

 piece by Mr. Tolles, optician, of Boston, U.S., consists in 

 mounting the prisms in a light material, vulcanite, 

 which are made to fit into the monocular microscope 

 body, taking the place of the ordinary eye-piece. The 

 image transmitted by the objective is brought to a focus 

 on the face of the first equilateral triangular prism by 

 the intervention of an erector eye-piece inserted beneath 

 it. The second set of prisms are by a rack-and-pinion 

 movement adjusted to suit any visual angle ; thus the 

 illumination of both fields is of nearly equal brightness. 



Spectro-Microscopy* 



The application of the spectroscope to the microscope 

 is one of the most beautiful additions the instrument 

 has ever received. The honour of the invention 

 appears to belong to H. C. Sorby, F.R.S., whose first 

 experiments were made with a simple triangular 

 prism, arranged and fixed below the stage, so that a 

 minute spectrum of any transparent object might be 

 readily examined, when placed in position imme- 

 diately before the slit. Shortly after the publication 

 of Mr. Sorby's paper, Mr. Huggins proposed to adapt 

 a direct vision spectroscope to the eye-piece, for the 

 purpose of viewing the spectra of opaque as well as 

 transparent objects. The exact form since adopted 

 is the Sorby-Browning Spectroscope. 



The first spectroscope made byMr.Browning(63 Strand) 

 is represented in fig. 73. A prism is placed at P, which is 

 enclosed in a box, so as to give a black field, by excluding 



