16 MOVEMENT 



While the needle is constantly rotating, we must 

 focus on the dial a camera provided with the shutter, 

 the exposure of which we wish to ascertain. The 

 opening of the shutter is effected by pressure on an 

 indiarubber ball. If the period of exposure is not 

 extremely short, the image of the needle will not be 

 clearly defined, but will occupy a more or less ex- 

 tensive segment of the dial. In such cases it would 

 be impossible to determine with any exactitude the 

 number of degrees occupied by the image. This is 

 due to the construction of the shutters, which give 

 incomplete illumination at the beginning and end 

 of the exposure.* 



From the blurred nature of the image, which 

 obscures the exact contour of the needle, one can only 

 approximately estimate that the distance travelled 

 during a single exposure is about three or four 

 degrees, which corresponds to about £$ of a second. 



It is almost impossible for a shutter, which gives a 

 single exposure, to produce one of very short duration. 

 For this object, the spring which moves the shutter 

 must be very powerful, and the weight carried ex- 

 tremely light. With shutters of this kind it is possible 

 to reduce the exposure to 2 ^ of a second. 



The shutters referred to in this work are of special 

 construction. They consist of fenestrated diaphragms 

 which, by means of continuous rotation, are able to 

 acquire immense velocity. Their fenestrations, moving 

 within the lens with extreme rapidity, produce a 

 succession of illuminations of very short duration. 



It is this which gives such good definition to the 

 images, which are shown in Fig. 12. Further, the 



* To obtain a true idea of the outline of the needle, it should 

 be r-ompared with the image of big. 12. In tin's ease the exposure 

 has been short enough to prevent any alteration in its shape through 

 the movement. 



