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. Befcre 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 with good cameras. The lens should be provided with an iris 
diaphragm capable of being stopped down to 128 or 256. The shutters 
furnished with the ordinary lenses are satisfactory, since “snap-shots,” 
tae . a cin Wiican adage 
Fig. 57. 5 x 7 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 carefully 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 
