46 



THE AMEEICAN MONTHLY 



[March, 



to form a perfectly correct idea of the 

 shape of any object by a few turns of 



the focussing screw. 

 Hence, persons whose 

 means are limited, and 

 who desire to invest a 

 small sum of money in 

 a microscope to be used 

 for purposes of study, 

 would do well to forego 

 anythoughtof purchas- 

 B A ing binocular stands. 

 Fig. 14. 

 On the other hand, there are certain 

 qualities of binoculars which com- 

 mend them to all workers who can 

 afford the additional cost. Apart 



Fig. 15. 

 from any stereoscopic effects, it is 

 doubtless true that the use of the two 



eyes whenever possible, renders con- 

 tinued observation less tiresome than 

 when only one can be applied to the 

 tube. Some writers have stated that 

 with a monocular, one eye is over- 

 strained while the other is not used 

 at all, contending that by using the 

 binocular that trouble is overcome. 

 The two eyes should be used alter- 

 nately with the monocular, hence they 

 both ought to become trained for 

 sharpness of vision; but we doubt if 

 the binocular aids in the way assumed, 

 for we are inclined to believe that, 

 although both eyes are simultane- 

 ously employed with the binocular, 

 the right eye does most of the real 

 work, the left eye only supplementing 

 its fellow and giving the binocular ef- 

 fect. However this may be, there is a 

 certain ease in working with binocu- 

 lars, which doubtless makes the strain 

 upon the eyes less, than with monocu- 

 lars. 



The stereoscopic effects while not 

 of great practical importance, as 

 already stated, certainly render many 

 objects more attractive to look at. 

 For this reason a microscope for 

 the entertainment and instruction 

 of friends should certainly be a bi- 

 nocular. But it should not be for- 

 gotten that the angular aperture of 

 the objective must not be too large, 

 as then the stereoscopic effect will be 

 exaggerated, as Carpenter has well 

 shown. He found that a ^-inch of 

 40° gave the true stereoscopic effect. 



It should be remembered that the 

 ordinary form of binocular cannot 

 be advantageously used with objec- 

 tives of less than half an inch focus. 

 Therefore, the kind of work to be 

 done should also be considered in 

 selecting the stand. 



We will now briefly refer to some 

 of the different forms of binoculars, 

 but a detailed account of them can- 

 not be given here. Most of the cuts 

 are taken from the article by Mr. 

 Geo. E. Fell, on " The Binocular Mi- 

 croscope and Stereoscopic Vision," 

 published in the Proceedings of the 

 American Society of Microscopists, and 



