FLIGHT. 109 



studies or, to put it more definitely, when the bird 

 will be represented about l/70th or more of its 

 life-size. 



* Movement ' in the Image. 



Exactly what speed is required for a definite 

 case will much depend on the nature of the result 

 desired and the purpose for which it is intended. 

 If we say that no photograph of a moving object 

 has ever yet been taken which did not possess 

 the blemish known as " movement," there may be 

 a host of workers up in arms against us, but un- 

 fortunately such an outcry would not affect the 

 truth of the statement. The theory of it is rather 

 important, as it directly touches on all instantane- 

 ous work. Suppose we took a photograph of some 

 uniformally moving object, a revolving wheel, for 

 example, and having given an exposure of l/50th of 

 a second, found the result was blurred. We then 

 tried a 1/1 50th of a second, but found there was still 

 a trace of " movement " in the spokes near the 

 circumference, and the next plate was given 1 /300th 

 of a second. Now the image was quite sharp, and 

 one might therefore feel inclined to say that l/300th 

 was the correct exposure for this subject ; but if 

 the picture were to be enlarged the blemish would 

 at once re-appear. If the movement in the first case 

 amounted to x part of an inch, then in the last 

 it would be l/6# of an inch, and no matter how 



