PHOTOGRAPHY, SCIENTIFIC ASPECTS 



graph and consider what happens. In this particular 

 moving-picture machine photographs are taken at 

 intervals of one-fifteenth of a second. 



Owing to the property of the retina, called "per- 

 sistence of vision," to which we have referred above, 

 a luminous impression cannot be instantly removed 

 but will persist, and hence affect the optic nerve, for 

 2/45 f a second, and be further prolonged, though 

 gradually weakening, for 2/45 of a second longer, 

 after the illuminated object has suddenly disappeared. 

 Now the moving pictures have been taken at 1/15 or 

 3/45 second intervals, and the various pictures are 

 exactly alike in so far as the stationary part of the scene 

 is concerned. A picture (let us call it image No. i) is 

 thrown on the screen and the opaque screen or shutter 

 of the machine then masks the light for 1/45 of a 

 second. Therefore, owing to the persistence of the 

 image, we shall see the picture not only during the 

 1/45 second of eclipse but also 1/45 second afterward. 

 But during the time of eclipse the next picture in the 

 film (No. 2) has been substituted for No. i, and con- 

 sequently when the light is again unmasked after the 

 1/45 second interval we shall see not only the image 

 No. i, somewhat weaker, though still distinct, and super- 

 posed on it is image No. 2. Since the stationary parts 

 coincide exactly, the eye perceives the sensation of the 

 moving object, the attitudes and positions in No. i being 

 succeeded by those of No. 2, and so until nine hundred 

 such impressions are made on the retina every minute. 



There is another method of presenting moving 

 pictures, in use in the familiar slot-machine, or muto- 



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