Ch. XII] HISTORY OF LENSES AND MICROSCOPES 433 



the binoculars of Nachet (1S53) and Harting (1858), the tubes are 

 parallel (§ 144). 



§ 698. Microscopes for two or more observers. — The projection 

 microscope with its real images on a screen has been commended from 

 the first invention of projection apparatus because many can see the 

 image at the same time, and the teacher or exhibitor can be sure that 

 the observers are seeing the special things he wishes to show. But 

 in looking into the microscope in the ordinary way only one person 

 can look at a time, even with the ordinary binocular. Therefore there 

 arose the effort to divide the light from the object so that two or more 

 could see the same image at the same time. The use of prisms for 

 dividing the light in the binocular gave the hint, and in 1853 Nachet 

 constructed a microscope for two observers, and another for three 

 observers (see figures of these in Harting and in Robin's work on the 

 microscope, also in the original paper). Harting, 1858, also produced 

 a microscope for two observers. For this the tubes were parallel. By 

 putting them closer together they served for a binocular for one person. 



Finally, in his enthusiasm for demonstration, he constructed a mi- 

 croscope in which the beam was divided among four diverging tubes 

 so that four persons could see the same specimen at once. 



Within recent years the demand for a way by which two observers 

 could look at once has given rise to two very practical double oculars 

 which are far enough apart so that two can look into the oculars con- 

 veniently. One was devised (1910) by Dr. Edinger of Frankfurt and 

 produced by Ernst Leitz in Germany, and the other in 19 16, by the 

 Spencer Lens Company of Buffalo, New York. In both these double 

 oculars there is an adjustable pointer so that the exact structure which 

 is to be studied can be indicated ; then both teacher and student can 

 be sure that they are talking about the same thing. 



§ 699. Oculars. — As shown above the first oculars were of single 

 lenses, — for the Dutch telescope-microscope a concave lens, and for 

 the Keplerian microscope a convex lens. 



For the Keplerian microscope, which soon became the only one 

 used for microscopic work, all sorts of experiments were tried both 

 for oculars and for objectives. Finally, about 1660, Huygens, the 

 great Dutch astronomer and physicist, designed for the telescope 



