LOW-POWER WORK 15 



the condenser and the object, for it causes a curiously granulated effect in the final 

 picture, which utterly spoils it. 



A compound condenser of at least 6-inch diameter, even when taking a quarter 

 plate negative, should always be' used. Its focus is of no material importance, but 

 should not be too short, for if so, the close approximation of the hme which then 

 becomes necessary may threaten the safety of the back lens, a matter of importance 

 in these large condensers, seeing that they are expensive. 



A question may now be asked. Of what advantage is the condenser, especially as 

 the light is so powerful ? When dealing with the use of a suhstage condenser to the 

 microscope further on, we fully explain the matter in its entirety, relating all that is 



Fig. 10 



necessary for the photographer to know, so a complete reply here seems unnecessary. 

 Still, to make the matter intelligible so far as required at the present moment, we may 

 briefly state the object of a condenser here is to gather light from the illuminant and 

 to place the rays in such a position that they may impinge on the object at the 

 proper angle ; and further, that by its means the object maybe illuminated uniformly 

 in all its parts. To the optical student the following explanation may still more 

 appeal. Consider Fig. lo. 



In Fig. lo A is shown the rays of light issuing in all directions from the 

 illuminant I. In Fig. lo B is seen a condenser C placed in such a manner that its 

 distance from I exactly equals its focus. The consequence is that rays of parallel 

 light issue from it, and these will continue parallel to infinity if all the optical condi- 

 tions of the condenser were perfect and the light a point. This arrangement is 

 required for certain purposes. In Fig. lo C the condenser is placed so that its 



