CHARACTERS OF ANIMALS. 127 



not SO much fatigued, as tlie image of the object appears 

 under a greater angle, and of an agreeable softness. But 

 when we attempt to employ this instrument for the 

 purposes of discovery, there is reason to complain of the 

 want of distinctness in the image, and the ill-defined ap- 

 pearance of its outline. Single microscopes have, there- 

 fore, obtained the preference among naturalists, and have 

 yielded all the most valuable information concerning minute 

 objects which we possess. They were employed by Leeu- 

 WENHOECK, Spallanzani, Ellis and Muller. The most 

 convenient form of the instrument, is the one which was em- 

 ployed by Ellis, (with the addition of rackwork to move 

 the stage and arm of the lens), and described by him in his 

 essay on Corallines. The ])ockei-^dL%s or Hand Megaloscopey 

 is the simplest form of the single microscope, and constitutes 

 a necessary part of the travelling apparatus (or rather daily 

 dress) of the zoologist. 



In the examination of objects with the microscope, it is 

 of importance so to manage the light, as to cause the rays 

 which illuminate the object proceed from one point. This, 

 in the case of the sun, is easily effected by making the rays 

 enter through a hole in the window-shutter, or, when a 

 lamp is used, the rays may be received through a hole in 

 a sheet of pastboard, and all the superfluous ones excluded. 

 The intensity of the light may be increased by a conden- 

 sing lens, or diminished by the interposition of plates of mi- 

 ca or glass. 



When animals are examined in water, it is frequently 

 a matter of considerable difficulty, to keep the lens from 

 touching the surface of the fluid, displacing the object, and 

 deranging the investigation. If the lens, however, be 

 fixed in its socket by means of wax, so as to prevent the 

 water reaching to the upper surface, it may be kept immers- 

 ed in the fluid, and niucli practical advantage obtained 



