PHOTOGRAPHY 



189 



enables small objects to be taken natural size and permits the use of a 

 telephoto lens. It should be provided with a swing and also a reversible back 

 by which the position of the plates can be changed instantly. The lens must 

 be of the telephoto pattern, which makes it possible to use the front or 

 back lens either alone or in combination. The chief advantage of this is 

 that the image, when distant, may be made of three different sizes without 

 changing the position of the camera. Generally speaking, the high-priced 

 rapid lenses are the best, since it is exceptional to get the desired length of 

 exposure in vegetation, on account of the ease with which the plants move 

 in the wind. Before buying such a lens it is desirable to test its rapidity 

 and depth of focus, since it is not necessarily better than some of the lenses 

 furnished wath good cameras. The lens should be provided w'ith an iris 

 diaphragm capable of being stopped down to 128 or 256. The shutters 

 furnished with the ordinary lenses are satisfactory, since "snap-shots," 



Fig. 57. 5x7 long focus "Premo" camera. 



i. e., instantaneous exposures, are practically never possible for plants. The 

 automatic shutter of the "Premo" camera is an especially convenient form. 

 All shutters should be carefilUy tested before using to determine the exact 

 time value of the exposures indicated. It is not uncommon for the exposure 

 at I second, or at other points, to have a value quite different from the one 

 indicated. When this is the case, it is evident that it can not be known too 

 soon. The camera should have at least a half-dozen double plate-holders. 

 These are numbered consecutively so that the figure uppermost when the 

 holder is in the camera will indicate the number of the plate exposed. A 

 carrying case is desirable on a long trip when all the plate-holders must 

 be taken, but ordinarily it is a disadvantage, since the camera box will carry 

 two or three holders. The camera cloth should be as small and light as 

 possible, and at the same time opaque. The most satisfactory one for the 



