N A CIIE^ S BINOCULAR MICROSCOPE. 117 



production of Stereoscopic effects will be produced ; but for cer- 

 tain classes of objects, this mode of exhibition is most admirably 

 adapted, the solid forms of the Potycystina (Chap.X), for example, 

 being brought out by it (especially when they are viewed as 

 opaque, not as transparent objects) with such a reality, as to 

 make them rese*mble carved ivory balls which the hand feels 

 ready to grasp. 



41. The same method of dividing the pencil of rays issuing 

 from the object glass, by a separating prism placed in its course, 

 has been applied by M. Nachet to another purpose, that of en- 

 abling two or more observers to look at the same object at once, 

 which is often a matter not only of considerable convenience, 

 but also of great importance, especially in the demonstration of 

 dissections. The account given by M. Cachet of the construc- 

 tion of this instrument, as adapted for two persons, will be found 

 in the " Quarterly Journal of Microscopical Science," Yol. II, 

 p. 72 ; he has subsequently devised another arrangement, by 

 which the form of the separating prism is adapted to divide the 

 pencil into three or even four parts, each of which may be di- 

 rected into a different body, so as to give to several observers at 

 one time a nearly identical image of the same object. Of course, 

 the larger the number of secondary pencils into which the pri- 

 mary pencil is thus divided, the smaller will be the share of light 

 which each observer will receive ; but this reduction does not in- 

 terfere with the distinctness of the image, and may be in some 

 degree compensated by a greater intensity of illumination. (See 

 Appendix for a description of American instruments and mo- 

 difications.) 



