ILLUMINATING THE OBJECT. 55 



beauty of the objects seen depends upon the management of the light 

 that is thrown upon or behind them, which can only be fully mastered 

 by practice. It may be remarked, however, as a general rule, that in 

 viewing those which are transparent, the plane mirror is most suitable 

 for bright daylight ; the concave for a lamp or candle, which should 

 have the bull's-eye lens, when that is used, so close to it that the rays 

 may fall nearly parallel on the mirror. 



If the bull's-eye lens is not used, the illuminating body should not 

 be more than five or six inches from the mirror. The latter is seldom 

 required to be more than three inches from the object, the details of 

 which are best shown when the rays from the mirror fall upon it be- 

 fore crossing ; and the centre should be, especially by lamplight, 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 achro- 

 matic condenser is employed instead of the diaphragm, 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 reflected 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 

 substitute. A large proportion of opaque objects are seen perfectly 

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

 as a background ; and for showing irregularities of surface, 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 com* 

 mand and variety. 



Mr. Ross very properly remarks, that the manner in which an ob- 

 ject is lighted is second in importance only to the excellence of the 

 glass through which it is seen. In investigating auy new or unknown 

 specimen, it should be viewed in turns by every description of light 

 direct and oblique, as a transparent object and as an opaque object, 

 with strong and with faint light, with large angular pencils thrown in 



