Ch. HI] BINOCULAR MICROSCOPES 83 



6. Pairs of oculars of different powers should be usable, for ex- 

 ample a pair of 5.x, a pair of iox oculars, etc., that is, the oculars should 

 not need to be special and should not be limited to one set. 



7. It should be possible to focus one tube independently to com- 

 pensate for difference in the two eyes of the observer. 



S. It should be possible to focus the entire microscope with one 

 coarse and one fine adjustment as for monocular instruments. 



9. The tubes may converge so that the axes of the eyes will be 

 directed to the near point of vision (250 mm.). 



10. The tubes may be parallel, then the axes of the eyes will be 

 parallel as for looking at distant objects. 



§ 143. Very early in the history of the telescope and of the com- 

 pound microscope, as nature has endowed us with two eyes, it was 

 insisted upon that both eyes should be used in examining objects in- 

 stead of using only one eye. This required two similar microscopes 

 or telescopes side by side and the right distance apart for the two eyes. 

 There still persists in the common opera-glasses the original binocular 

 Dutch telescope-microscope. 



The modern binocular dissecting microscope with two tubes and 

 two objectives and two oculars is in principle like the original binocular 

 microscope of Cherubin d'Orleans (1677), except, of course, the earlier 

 one had no erecting arrangement. 



The double microscope with two complete tubes, two objectives, 

 and two oculars is not available for high powers, for the two objectives 

 cannot be close enough together to bring the exceedingly small object 

 into the field of both microscopes at the same time. Naturally, 

 therefore, an effort was made to use a single objective and to divide 

 the light passing through it so that half should go to the right and half 

 to the left eye. The first successful binocular of this kind was invented 

 by Riddell of New Orleans in America in 185 1. In this, four prisms 

 are used just above the objective and serve to divide the light equally 

 and to pass it on to the two eyes through two parallel tubes, each with 

 its own ocular. Later a satisfactory form was invented by Mr. 

 Wenham of England in which there is but a single prism (fig. 52). 

 Neither of these forms permitted of very high powers. Finally, in 

 1864, Mr. Robert B. Tolles invented a binocular, stereoscopic eye- 



