MICROSCOPES. 207 



centre, as in Fig. 4, which you can see distinctly is nothing 

 but the image of the hole in the diaphragm, seen more or less 

 in focus according to the adjustment. If you take a diaphragm 

 with a central stop, you see what looks very strange, viz., a 

 bubble or spherical cavity with a central black spot, from 

 which proceed the two small arms, as shown by Fig. 5. This 

 appearance is thus manifestly due to nothing but the image 



O 



FIG. 3. FIG. 4. FIG. 5. 



of the opening in the diaphragm, the central stop, and the 

 two arms which support it. These very simple facts thus 

 clearly prove that what might easily be mistaken for structure 

 may be merely the light and shade depending on the kind 

 of illumination made use of. This way of viewing the sub- 

 ject has occurred to me only quite recently, and I feel 

 persuaded that an arrangement which gives us the means of 

 limiting the obliquity of the light and also the size of a 

 more distant opening is very useful, and would be equally 

 applicable in the case of rods, and minute fibres, and such 

 kinds of structures as are commonly met with in organic 

 bodies. In the case of the markings on some diatoms, we 

 do indeed see all the appearances that would be due to a 

 vast number of small lenses arranged side by side ; and, as 

 in the case of the above-described cavities, we may so modify 

 the illumination as to show the external outlines of these 

 bead-like lenses, or to see merely a central black spot in each 

 of them. As a general rule, however, the curvature of the 

 surfaces of organic bodies are seldom sufficiently regular to 

 give distinct images of the openings in the sub-stage 

 diaphragm, but yet there can be little doubt that the lights 

 and shades are to a very great extent due to a similar 

 cause, and that as far as this is concerned they might be 

 looked upon as very much distorted and irregular spherical 

 or cylindrical lenses. In any case, much more may be learned 

 of their true form by carefully observing and discussing the 

 changes that take place, on varying the divergence of the 



