216 THE BOOK OF THE LANTERN. 



normal length. The third modification which I have intro- 

 duced, is a groove cut in the milled head of the fine adjust- 

 ment screw, the purpose of which we shall presently see. 

 For photographic work, the mirror is thrown out of gear, 

 as indicated in the diagram, for it is not required. 



Referring once more to this diagram, 1 1 is the table on 

 which the whole arrangement rests, and it is best to have 

 a table for the purpose, or at any rate a level base board? 

 upon which all necessary fixtures can be made; c is a 

 camera which should open out to great extent (most 

 modern cameras are made to do so, so as to give the 

 photographer the benefit of using long-focus lenses) ; 

 m is the microscope, s the stage, p the mirror thrown 

 back out of use, n the coarse adjustment, and k the fine 

 adjustment. We can now see the advantage of providing 

 this milled head of the fine adjustment screw with a 

 groove. In this groove is placed a silk cord, which 

 works in a corresponding groove in the little wheel 

 which is fixed on the end of the focussing rod o o. By 

 means of the knob d at the other end of this rod, the 

 operator is able to work the fine adjustment to a nicety, 

 while he is far away from the microscope, examining 

 the image on the ground-glass screen of the camera. 



A word about this screen will not be amiss. Ordinary 

 ground-g'ass will not do for this class of work, for it is 

 far too coarse. It should therefore be replaced by a 

 focussing screen prepared as follows : Obtain a sensitive 

 gelatine plate, such as is used for negative work, expose 

 it to the light of a gas flame for a second, and then pro- 

 ceed to develop it. Develop until it is uniformly 



