HOW TO USE THE MICROSCOPE. 53 



is seldom required to be more than three inches from the 

 object, the details of which are usually best shown when the 

 rajs from the mirror fall upon it before crossing, and the 

 centre should' (especially by lamp-light) be in the axis of the 

 microscope. For obscure objects, seen by transmitted light, 

 and for outline, a full central illumination is commonly best ; 

 but for seeing delicate lines, like those on the scales of insects, 

 it should be made to fall obliquely, and in a direction at right 

 angles to the lines to be viewed. 



" The diaphragm is often of great use in modifying the light, 

 and stopping such rays as would confuse the image (especially 

 with low or moderate powers), but many cases occur when the 

 effects desired are best produced by admitting the whole from 

 the mirror. 



" If an achromatic condenser is employed instead of the dia- 

 phragm, its axis should correspond with that of the body ; and 

 its glasses, when adjusted to their right place, should show the 

 image of the source of artificial light, or, by day, that of a 

 cloud or window bar in the field of the microscope, while the 

 object to be viewed is in focus. 



" The most pleasing light for objects in general, is that re- 

 flected from a white cloud on a sunny day ; but an Argand's 

 lamp or wax candle with the bull's-eye lens is a good substi- 

 tute. 



" A large proportion of opaque objects are seen perfectly well 

 (especially by daylight) with the side-reflector, and the dark 

 box as a background ; and for showing irregularities of sur- 

 face, this lateral light is sometimes the best; but the more 

 vertical illumination of the Lieberkuhn is usually preferable, 

 the light thrown up to it from the mirror below being, with 

 good management, susceptible of much command and variety." 



The management of the light with opaque objects must 

 depend in a great degree upon their size, and the manner in 



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