APPARATUS AND MATERIALS. 21 



in the leisurely fashion suggested, instantaneous 

 work would be a matter of some difficulty. A small 

 point, but an important one, is to have the shutters of 

 the dark slide self-locking. Spirit levels, fancy move- 

 ments, and so forth, are merely in the way. An 

 endeavour should be made to secure apparatus of such 

 a simple character that the hands can be trained to 

 manipulate it automatically, while the eyes are 

 employed elsewhere. 



The writer is unfortunately unable to enter on the 

 vexed question of lenses with any degree of authority 

 or experience. All the work reproduced in the present 

 volume was done with an unpretentious French rapid 

 rectilinear, for which he was recently offered 75. 6d. 

 exchange value by a well-known City firm. This lens 

 has been found quite fast enough for ordinary purposes. 

 For objects in the same perpendicular plane it covers 

 a half-plate at full aperture f/8. The depth of focus 

 is, of course, increased by stopping down, and as 

 animals are of three dimensions, copying small ones 

 so that they shall be of a reasonable size on the plate 

 invariably necessitates considerable stopping down and 

 increased exposure. The inventor of an optical 

 system which shall copy a lizard end on at a large 

 aperture, so as to show sharp definition from head 

 to tail, will earn the undying gratitude of the 

 naturalist. 



The shutter is the naturalist's principal difficulty. 

 After experimenting with every form on the market, he 

 is not unlikely to resign himself to the simple flap. 



