260 SUPPLEMENT TO THE 



be taken never to view them both as opaque and transparent 

 bodies at the same time, as this is sure to produce decep- 

 tion, and on this account they should be shaded from 

 all incidental rays. Their true colours can never be seen 

 properly by high powers : in order to ascertain them cor- 

 rectly., a low power should be employed, with the light of 

 the sun reflected by a plaster of Paris disc. 



Observers should study the effect of intercepted light on 

 ordinary transparent bodies, of the real nature of which 

 there can be no doubt, in order to be able truly to appre- 

 ciate microscopic phenomena. There are a number of 

 glass toys made, which will make very good subjects of 

 this description ; and so do ordinary glass tubes, and solid 

 rods of glass the former being filled with, other trans- 

 parent objects, and afterwards with water. We frequently 

 meet with bodies in transparent objects which operate 

 like lenses on surrounding objects, of which they form 

 miniature images, very remarkable to those who are not 

 aware how the effect is produced. 



In short, we must consider, that in all bodies viewed 

 by intercepted light, there is, properly speaking, neither 

 light nor shade, in the ordinary acceptation of these terms ; 

 there are only dark and light parts, which again assume 

 new aspects as the light is more or less direct or oblique. 

 Thus depressions on transparent objects are almost sure, 

 under the action of oblique light, to assume the effect of 

 prominences ; but prominences seldom or never the sem- 



therefore made a section of it, and examined it as an opaque body, and 

 thought that I ascertained in this way that the lenses had become concave 

 instead of convex, from collapse, or from the drying up of the substance 

 between their exterior cases or laminae. I have attempted to treafc lined 

 objects in this way, but could never succeed in developing their mysterious 

 tissue, owing to their extreme minuteness. 



