30 THE MICROSCOPIST. 



third or fourth classes of instruments are intended for 

 popular and educational use, and are fitted not only with 

 stands of more simple workmanship, but with cheaper 

 lenses, although often very good. Some French achro- 

 matic objectives, adapted to this class, are suitable for all 

 but the very finest work. 



Binocular Microscopes. The principle of the stereoscope 

 has been applied to the microscope, so as to permit the 

 use of both eyes. The use of such an instrument with 

 low or medium powers is very satisfactory, but is less 

 available with objectives stronger than one-half inch focus. 

 There are two ways of accomplishing a stereoscopic effect 

 in the microscope. The first and most common is by 

 means of Wenham's prism (Fig. 5), placed above the ob- 

 jective, and made to slide so as to transform the binocular 

 into a monocular microscope. 



The second mode is to place an arrangement of prisms 

 in the eye-piece, so as to refract one-half the image to the 

 right and the other half to the left, which are viewed by 

 the corresponding eyes. In either construction there is a 

 provision made for the variable distance between the eyes 

 of different observers. In the frontispiece is a representa- 

 tion of Zentmayer's grand American microscope, which 

 will afford a good idea of the external appearance of a 

 first-class binocular microscope. Students' and third-class 

 microscopes, as before said, are less complicated and of 

 more moderate cost. The mechanical and optical per- 

 formance of Zentmayer's large instrument leaves scarcely 

 anything to be desired. Instead of the more expensive 

 rack-work stage, a simple form, originally invented by Dr. 

 Keen, of Philadelphia, and copied by Nachet and others, 

 is often employed. It consists of a rotating glass disk, to 

 which is attached a spring, or a V-shaped pair of springs, 

 armed with ivory knobs, which press upon a glass plate 

 in the object-carrier. The motion is exceedingly smooth 

 and effective. 



