212 LECTURES TO SCIENCE TEACHERS. 



the kind of object-glass ought to depend on the kind of work 

 we wish to do. If we want to examine the very minute 

 structure of thin flat objects we must use one kind, and if 

 we want to look at things through a considerable thickness 

 of water or glassy matter, and to study various irregular 

 objects met with in original research, we must have glasses 

 which do not approach too near the object, even if the angle 

 of aperture and the definition of very close markings are by 

 this means somewhat diminished. 



In connection with the visibility of very minute objects 

 there is another interesting point. If in order to define objects 

 that are very close together you increase the angle of the 

 divergence of the light, you then bring into play another prin- 

 ciple. I have already described how you may thus obliterate 

 the object, by destroying all difference in shading. We Come 

 then to this dilemma, that if the aperture is small we cannot 

 see the object, for one reason ; and if we have the aperture 

 large we cannot see it for another reason. In one case the light 

 would break down, and in the other the object would not 

 possess any character which would enable us to- see it, and 

 thus with these very high powers we come across a state of 

 things which makes it exceedingly difficult to go much beyond 

 what we have obtained. In fact, I am much inclined to 

 believe, that as far as the size of the object is concerned, we 

 have pretty nearly got to our limit. I hope I may be wrong, 

 but I am very much afraid, that except under special condi- 

 tions, we can never see objects of as minute a size as would 

 be very desirable in studying certain characters of natural 

 history, and that our powers are limited by the constitution 

 of light itself. Perhaps something may be done to increase 

 our power by not using simple transmitted light. 



This lecture would be incomplete if I were not to say 

 something about the difference between the monocular and 

 binocular microscopes. The disadvantage of monocular 

 instruments is of course that we use only one eye. We injure 

 one eye with using it too much, and the other by not using 

 it enough, and we also do not have the stereoscopic effect. 

 These disadvantages led to the desire to contrive some method 

 by means of which we could make use of both eyes. In the 

 Exhibition you may see a magnificent collection, lent by Mr. 

 Crisp, including, I believe, every kind of binocular instrument 

 that has been contrived. Some of these are so uncommon 



