116 



CONSTRUCTION OF THE MICROSCOPE. 



Fig. 31 is shown the Binocular Microscope constructed by M. 

 FIG. si. Nachet upon this plan. The arrangement 



of the base and stage is that commonly 

 employed in French vertical Microscopes ; 

 and a stem rises from the back of it, with 

 which the double body is connected by a 

 rack-and-pinion movement that gives the 

 focal adj ustment. The apparatus of prisms 

 shown in Fig. 30, is placed between the 

 object-glass and the lower ends of the 

 bodies ; and by means of a double-threaded 

 screw acted on by a milled head between 

 the two bodies, they may be separated 

 from, or approximated towards, each other; 

 so that the distance between their axes 

 may be brought to coincide with the dis- 

 tance between the axes of the eyes of the 

 individual observer. The author can con- 

 firm by his own experience the statement 

 of M. Cachet, that this instrument is en- 

 tirely free from that tendency to produce 

 pseudoscopic effects, which is the great 

 drawback in Prof. Riddell's and in Mr. 

 Wenham's arrangements ; and it comes so 

 near the theoretical standard of perfection, 

 when used with low powers of moderate 

 Binocular Microscope, aperture, that its performance may be con- 

 sidered highly satisfactory. Its definition, however, when used 

 with higher powers of larger angular aperture, has not yet been 

 rendered sufficiently good, to enable it to afford a satisfactory 

 view of the more difficult class of test-objects; and it maybe 

 doubted whether, considering the number of deflections which 

 the rays undergo in their course, such perfect definition is to be 

 anticipated. For although their general course on entering and 

 emerging from each prism may be perpendicular to its surfaces, 

 so that they suffer no refraction, many of them will be slightly 

 oblique, and will therefore undergo not only refraction, but also 

 some amount of chromatic dispersion. And it is moreover to be 

 recollected, that when high powers are being employed, and 

 especially such as are of large angular aperture, the smallest de- 

 parture from exactitude in the focal adjustment gives indistinct- 

 ness to the image. Now the special object of this instrument 

 being to convey to the mind the notion of the solid. forms of 

 objects, of which some parts project more than others, it is obvi- 

 ous that the rays proceeding from the projecting parts cannot be 

 so. nearly brought to the same focus with those from the receding, 

 as to produce an even tolerably distinct image of both at once. 

 It seems likely to be only with objectives of comparatively low 

 power and small angular aperture, that images suitable for the 



