BINOCULAR MICEOSCOPES 



97 



and it is quite immaterial, as regards this result, which portion of the 

 emergent rays is admitted by tJie right or the left part of the objective. 



The manner in which the delineating pencils are transmitted 

 through the system may he such as to require crossing over of the 

 rays from the right-hand half of the objective to the left eye-piece, 

 and vice versa. But it is not essential to binocular effect. In the 

 Weiiham and Xacliet binocular (pp. 98, 99) crossing over is required 

 because the inversion of the pencils js not changed by two reflexions. 

 If the delineating pencils ha ve l.een reflected uneven number of times 

 in the same plane, it will be necessary for the rays to cross ; but if 

 they have been reflected an odd number of times, it is not only un- 

 necessary, but is destructive of orthoscopic effect, provided ordinary 

 eye-pieces (non-erecting) are employed. Hence in the Stephenson bi- 

 nocular it is not only not required, but would give pseudoscopic effect. 



Principal Forms of Binocular Microscopes. The first binocular 

 of a, practical character was the arrangement of Professor J. L. 

 lliddell, of Xew Orleans. It was devised in 1851 and constructed in 

 1852, and a description of its nature and its genesis was given by 

 him in the second volume of the first series of the ' Quarterly Journal 

 of Microscopical Science' in the year 1854. 1 



A representation of his original instrument is presented in fig. 71 , 

 and the arrangement of the prisms by which the binocular effect was 

 obtained is shown in fig. 72. 



It will be seen that the pencil of rays emerging from the back 

 lens of the combination I is divided into two, each half passing re- 

 spectively into the right and left prisms ; the path of the rays is 

 indicated at a, b, c, d, the object being at o. 



To secure coincidence of the images in the field of view for 

 varying widths between the eyes Professor Riddell devised (1) a 

 means of regulating the inclination of the prisms by mounting them 

 in hinged frames, so that, while their lower edges, near a, fig. 72, 

 remain always in parallel contact, the inclination of the internal 

 reflecting surfaces can be varied by the action of the milled head in 

 front of the prism box ; (2) the lower ends of the binocular tubes 

 are connected by travelling sockets, moving on one and the same 

 axis, on which are cut corresponding right- and left-handed screws, 

 so that the width of the tubes may correspond with that of the 

 prisms ; and (3) the upper ends of the tubes are connected by racks, 

 one acting above and the other below the same pinion, so that right- 

 aiid left-handed movements are communicated by turning the pinion. 



This instrument could only be used in a vertical position, as 

 shown in the figure (71). The two prisms in fig. 72 correct the in- 

 version of the image in a lateral direction, two more prisms are 

 needed to correct the inversion in the vertical direction. These 

 Professor lliddell placed above the eye caps, but now they are placed 

 immediately above the binocular prisms, fig. 78. 



This system of binocular excited much interest in England im- 

 mediately after its publication, and Mr. Wenham in London and 

 MM. Nachet, of Paris, soon suggested and devised a variety of 

 binocular systems. 



A P. 13. 



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