258 SUPPLEMENT TO THE 



under a variety of methods of illumination, conjointly 

 with a consideration of the phenomena presented by 

 bodies when seen in perspective, or, as painters term it, 

 foreshortened. We are particularly perplexed in micro- 

 scopic observations, by the circumstance that only a point 

 of an object can be seen at a time, if the power used is 

 considerable ; we must, therefore, always begin to study 

 them with low powers at first, and increase them gradually, 

 so that we may always be able to recognize the particular 

 part of the object we are looking at, otherwise all will be 

 confusion. When we see an opaque object represented 

 by a drawing, we see it as it never can be seen by a micro- 

 scope, because, in the latter instrument, its various 

 features can never be in focus at the same time, as before 

 observed, and it would be impossible to represent it on 

 paper exactly as seen at a variety of adjustments, without 

 an infinite number of views of it. 



I particularly recommend observers, in examining and 

 verifying opaque objects too small to be dissected, to use 

 the simple light of a candle before the stage, as its diver- 

 gent rays bring out more strongly their various component 

 features, and render them more intelligible than can be 

 done by any other method. The oblique light of the taper 

 plays over their various prominences and depressions like 

 that of the evening sun over common objects, giving 

 broad lights and shadows; and, if the observer has any 

 knowledge of the operation of light, and the manner in 

 which it is broken and intercepted by ordinary bodies, he 

 will hardly fail to arrive at a tolerably clear idea of the 

 nature of the minutiae he is contemplating. The light of 

 the sun must never on any account be used, as it gives 

 rise to an infinity of indescribable deceptions, both as to 



