378 E. M. NELSON ON BINOCULAR MICROSCOPES. 



You will then naturally ask, If these new binoculars are not 

 stereoscopic, what is their use? Their use is confined to the 

 employment of full Kamsden discs in each eye, that is for 

 work with non-stereoscopic images. An enormous amount of 

 microscopic work is done with images of that kind, and when 

 prolonged work is undertaken with the new binocular great 

 relief and comfort to the eyes will be secured. But to say, on 

 the one hand, that one of these instruments when used for, 

 say, the examination of pond life with a \ inch and the Mercer 

 effect is going to supersede a Wenham, and on the other hand 

 to state that by means of this new binocular delicate secondary 

 structures on diatoms will be more easily seen than with a 

 monocular, is to talk nonsense. At the upper limit they cannot 

 compete with the monocular, and at the lowest limit they cannot 

 compete with the Wenham ; but in their own sphere they are 

 extremely useful and form a very important addition to the 

 modern improvements in Microscopy. 



At any time with the new binocular the Mercer effect can be 

 turned on to determine the relation of the various parts of an 

 object ; but it must be borne in mind that stereoscopism in a 

 microscope with the higher powers is only partial, and whether 

 it is present or not depends largely upon the nature of the 

 object; for example, with a medium power, such as \ or a 

 i, the rays of a Heliopelta will exhibit strong stereoscopism, 

 but many other objects with the same power will show none. 

 With a \ and a spread slide of P. angulatum, it is difficult to 

 determine whether a valve is convex or concave side up. Stereo- 

 scopism in macroscopic vision differs from that in microscopic 

 vision inasmuch as it is influenced greatly by the thickness of 

 the object. 



With macroscopic vision stereoscopism is seen equally well 

 with either a book or a bookcase, but that is not so with 

 microscopic vision. In that case stereoscopism would be present 

 with our allegorical bookcase but not with the book. Low 

 powers deal with thick, coarse objects, and therefore stereoscopism 

 is present ; but with the higher powers it is necessary to select 

 suitable objects for the demonstration of the stereoscopic effect. 

 For instance, bacteria dried on cover do not exhibit any more 

 stereoscopism with the new binocular than with a monocular, 

 for in a monocular they can be made to look like sausages ; but 



