376 E. M. NELSON ON BINOCULAR MICROSCOPES. 



amount is difficult to determine, nor is it known if it is the 

 same for all persons. As I pointed out elsewhere, it is inaccurate 

 to say that there is an increase of apparent magnification in a 

 binocular ; what really takes place is that in a monocular there 

 is a diminution of apparent magnifying power, and that this dimi- 

 nution is non-existent in a binocular. If any one examines a 

 lighthouse, a ship, or other object with a 2 or 3 power monocular 

 telescope, the image appears no larger than when it is seen with 

 the naked eye. The image, as any one will tell you, is brighter 

 and clearer, but not larger. Directly the image seen in the 

 telescope is superimposed on that seen with the other eye the 

 magnification of the monocular is demonstrated, which generally 

 causes surprise. Having given this subject considerable attention, 

 I am of opinion that the true magnification is seen in a binocular, 

 but that with a monocular, either telescope or microscope, this 

 is reduced. 



The third attribute viz. illumination : It is doubtful if there 

 is much gain in the Greenough type of binocular, as the amount 

 gained by the use of both eyes is probably lost owing to 

 the prisms, surface reflections, etc. Of course, with a single 

 objective type of instrument there must be a loss. This is of 

 no importance, for in a microscope one has usually more light 

 than is needed. 



The fourth attribute : Experiments have shown that colour 

 tints are increased in a binocular ; this is a distinct gain, for 

 there is always much and often total loss of colour in micro- 

 scopical observations. 



There is another form of binocular which must be mentioned, 

 viz. the binocular eye-piece. This was an early invention of 

 Wenham ; the next to take it up was Tolles, of Boston, U.S.A., 

 who made a very good one by using prisms on the Nachet 

 plan, dividing the beam by means of an isosceles prism. Tolles' 

 binocular was well made, stood deep eye-pieces, and had the 

 advantage that both tubes were similar ; consequently the illumi- 

 nation and path of the rays was equal in each. The advantage 

 this system possesses is that it permits of objective correction by 

 draw tube. With other binoculars objective correction is not so 

 easily accomplished. 



The last form of binocular eye-piece was brought out by 

 Professor Abbe. This, as we have seen above, was a failure. 



