NACHETS BINOCULAR MICROSCOPE. 



115 



That such a method would produce the Stereoscopic effect, might 

 be anticipated from the result of the very simple experiment of 

 covering the right-hand or the left-hand half of an object-glass of 

 low power, during the examination of any object that lies in 

 oblique perspective ; for the two views of it thus obtained, will 

 be found to present just the kind and degree of difference which 

 is observable in stereoscopic pictures. The first attempt to put 

 this plan into execution, seems to have been that of Prof. Riddell, 

 of New Orleans ; but the results of his method, as followed by 

 opticians on the European side of the Atlantic, were far from 

 answering the expectations excited by his own description of 

 them. The subject was both theoretically and practically inves- 

 tigated by Mr. Wenham, with much ability (Transactions of the 

 Microscopical Society, new series, Vol. II, p. 1) ; and a Binocular 

 Microscope on a pattern suggested by him, was constructed by 

 Messrs. Smith and Beck. This, too, was far from satisfactory in 

 its performance, having two capital defects ; namely, first, that 

 the view which it gave was often pseudoscopic^ the projecting por- 

 tions of the object appearing to be depressed, and vice versd ; and 

 second, that the two bodies being united at a fixed angle of con- 

 vergence, the distance between their axes could not be con- 

 veniently adapted to the varying distances of the eyes of different 

 individuals. The construction adopted by M. Cachet, however, 

 is much more successful. His method is to divide the pencil of 

 rays issuing from the objec- 

 tive, by means of a prism 

 (Fig. 30, p) whose section is 

 an equilateral triangle ; for 

 the rays a b on the right side, 

 which enter the flattened sur- 

 face presented to them, are 

 reflected, by impinging very 

 obliquely against one of the 

 internal faces of the prism, 

 towards the left, emerging 

 again from the prism, as they 

 had entered it, almost at right 

 angles ; and in like manner 

 the rays a f b f on the left side 

 are reflected towards the 

 right. Each of these pencils 

 is received by a similar prism, 



Which again Changes itS direC- Arrangement of Prisms in Nachet's Binocular 



tion, so as to render it parallel 



to its original course ; and thus the two halves a b and a' V of 

 the original pencil are completely separated from each other to 

 any interval that may be required, this interval being determined 

 by the distance between the central and the lateral prisms. In 



FIG. 30. 



