CAMERAS AND PICTURE-MAKING 313 



to match the dark strip beside it. A circular strip of the face 

 adjacent to the rim can be turned as the rim is rotated. On 

 this strip are marked a series of numbers indicating diaphragm 

 sizes and the sensitiveness of various plates. On the edge of 

 the central disk, a series of numbers indicates seconds and frac- 

 tions of a second. Accompanying the exposure meter is a 

 booklet giving the sensitiveness of various makes of plates. 

 Suppose we are using Cramer's instantaneous isochromatic 

 plates. The booklet gives its speed as F.m, which means that 

 this plate would require an exposure of one second with a dia- 

 phragm opening of F.m under standard conditions. Suppose 

 that it has required three seconds for the strip of sensitive paper 

 to darken. Then set 3 on the central disk opposite F.m on 

 the circular strip. One may now read the exposure required 

 for any diaphragm in seconds or fractions of a second. Thus 

 if one is going to use an F.64 diaphragm opening he will give an 

 exposure of one second, or if he wishes a short exposure, say one 

 sixty-fourth of a second, he must use the F. 8 diaphragm opening. 

 The sensitive paper rotates when the back of the instrument is 

 turned, thus bringing a fresh bit under the opening for the next trial. 



Since the enlargement of the diaphragm opening means the 

 admission of more of the confusing marginal rays, the rule is 

 to use as small a diaphragm opening as possible. For motionless 

 objects one will use say an F.64 stop, and give a long exposure. 

 But for rapidly moving objects, or even slowly moving ones, 

 when the light is dim one must use a large stop and give a short 

 exposure. Under such conditions a well-corrected lens must 

 be used. The cheaper grades of cameras are therefore not made 

 with large diaphragm openings. 



The procedure in taking the picture, then, is as follows. Set 

 the camera firmly on its tripod, and point it at the object. Open 

 the diaphragm wide, and focus so as to get a clear image on the 

 ground glass, the desired object at about its center. In the 

 better cameras the lens board may be raised or lowered to facili- 

 tate such centering without moving the tripod. The back carry- 



