ON THE ILLUMINATION OF OBJECTS. 187 



light will be obtained. In both these latter cases there will be 

 a larger field of view illuminated, but with a great diminution 

 of its brightness; hence the use of the mirror being made to 

 slide. For all objects of large size, such as sections of wood, 

 mosses, the wings of insects, &c., where low powers only are 

 required, the illumination may be effected by the concave 

 mirror, which, for the purpose, should not be less than two 

 mches in diameter. Nothing is more simple than to illu- 

 minate a large object under a low power, but difficulties will 

 be found when high powers are used ; and it is important that 

 all the light possible should be made available for the purpose; 

 it becomes necessary, therefore, to have recourse to the other 

 optical contrivances presently to be described. 



To cut off all superfluous light, an apparatus termed the 

 diaphragm, described at page 111, fig. 56, is generally fitted 

 to the under side of the stage ; it is supplied with a moveable 

 plate, in which there are three or more holes of different 

 sizes; each of these in succession may be brought between 

 the mirror and the object, and it will be found advisable to 

 try by which of the apertures the best definition is effected. 



Some persons recommend the interposition of a plate of 

 ground-glass between the mirror and the object, to diffuse the 

 light equally over the field, and to destroy all the glare occa- 

 sioned by the reflection of a large body of light ; but this is 

 only applicable to objects of large size, where low powers are 

 used. When applied to the higher powers, and to objects 

 which require to be well defined, the field of view is then 

 rendered so misty, that all sharpness of outline is destroyed. 

 In this case a great improvement is effected if the groimd 

 surface of the glass be made greasy, which allows more light 

 to be transmitted. Another way of softening the glare is to 

 supply the place of the ground-glass with glass of a neutral 

 or blue colour ; this is recommended to destroy the reddish- 

 yellow colour given off from all lamps or candles, and to make 

 the artificial light as pleasant to use as that of the natural or 

 daylight ; but none of these appliances should be used with 

 high powers, or where accurate definition is necessary, as in 

 the case of test objects. 



