196 MICROSCOPIC OBJECTS. 



matic, and should be so constructed that they will act either 

 together or separately. A stop |^ of an inch in diameter 

 is made to slide into the exterior tube, and to be removed 

 at pleasure. This will in no degree diminish the size of 

 the visual pencil or penetrating power of the glasses, when 

 they are all in combination, though it greatly reduces the 

 quantity of light, or what may be termed the intrinsic 

 brightness of the image, while it increases wonderfully its 

 distinctness ; and for this purpose it may be advisable to 

 use it occasionally, though the combination is abundantly 

 distinct without it on any opaque objects not of extraordi- 

 nary brilliancy, such as globules of mercury, diamond 

 beetles, and certain minerals. I need not observe, that 

 wAen the two glasses in front at d, only are employed, the 

 said stop reduces their aperture, and with it the size of the 

 visual pencil and the penetrating power. The focus of the 

 body is adjusted by moving the milled head at e back- 

 wards and forwards ; the entire body may also be moved 

 in its containing tube f if micrometers are applied to 

 the field-bar, whose value it is not wished to disturb. 



As this instrument has a very shallow eye-piece, its 

 visual pencil is of considerable length ; thus camera luci- 

 das, rectangular and other prisms,* plane metals, at 

 different angles, may be applied comfortably to it without 

 preventing us from seeing the whole field of view, as they 



* A prism of 60° throws out a pencil at a very convenient angle 

 for observation ; and the visual pencil of a megaloscope is long 

 enough to permit the employment of it, and allow the whole field 

 of view to be seen at once. That portion of the lower part of it, 



