THE COMPOUND MICROSCOPE 



stage in such a manner that, as it is revolved, holes 

 of various diameters are brought one by one within 

 the cone of light. It need hardy be remarked that 

 the smaller the hole in the diaphragm the more 

 light is cut off and the less reaches the object. In 

 more modern instruments and in practically all 

 which are fitted with a condenser, an Irisjdiaphragin .^ 

 is fitted. A diaphragm of this nature consists of 

 a number of thin, blackened, metal leaves, fastened 

 to a metal ring in such a manner that, when the 

 ring is revolved, the leaves close together, making 

 the opening in the centre smaller and smaller. The 

 ^^Y^ I^is diaphragm has many advantages over the old 

 ^'f( ^ perforated metal plate. At will, we can have any 

 opening from full to the merest pin-point or we can 

 cut off the light rays altogether, should we wish to 

 do so ; we are not confined to a definite number of 

 stops. As we cut off these outer rays of light we 

 shall find that, up to a certain point, though the 

 illumination becomes less and less the object be- 

 comes more and more clear, or, to use the correct 

 expression, its definition is improved. 



When it is necessary to cut off some of the central 

 rays of the light cone, either a circle of glass with 

 an opaque centre is dropped into a metal holder 

 below the stage, or a circular metal plate, held in 

 the centre of a metal ring by three arms, is used 

 in the same manner. 



The effect of cutting off the central rays of the 

 Ught cone is, of course, to reduce the illumination | 

 and to show up delicate detail to advantage. No 



62 



