204 MICROSCOPIC ILLUSTRATIONS. 



retain it firmly, so that it can be twisted about in any 

 direction, without risk of detaching it. This will be 

 found a most useful contrivance for holding the cork 

 cylinders, and all objects whatsoever mounted on pins. 

 The swivel which carries the nippers, &c. is thrust into 

 the arm, and in that manner used, as shewn in the plate. 



The mode of observation by plain artificial light 

 corresponds with that by plain daylight. It is one of 

 the simplest and best ways of viewing opaque objects ; 

 and the contrast of the strong divergent lights and 

 shadows cast by the rushlight, brings out, and enables 

 us to comprehend, the foreshortenings and perspective 

 of the complicated details frequent in irregular opaque 

 subjects, and thus to verify their true nature, construc- 

 tion, and relative position, much better than, I think, 

 any other sort of illumination. 



Never forget that, when observing with the candle 

 before the stage, you must hold your breath, or have a 

 handkerchief tied over your mouth : a single puff is all 

 that is necessary to cause the flame of the light to play 

 over the object and destroy it (and thus in fits of absence 

 have I destroyed many valuable ones). What renders a 

 rushlight so useful is, that it will not, of itself, flare over 

 your object : it can, moreover, on account of its small- 

 ness, be got closer to an object with impunity than any 

 other artificial light ; and thus, in fact, gives the most 

 intense sort of plain illumination; for the intensity of 

 light decreases according to the square of its distance 

 from the object. 



