64 MOVEMENT 



difficulty, however, can be overcome by a variety of 

 expedients, one of which is already known to us, and 

 which depends on the horizontal and forward move- 

 ment of the photographic plate. 



In speaking of the photographic registration of 

 the variation of Lippmann's electrometer, we showed 

 how successive images of the illuminated column of 

 mercury formed a continuous series, owing to the 

 onward movement of the plate. This mode of repre- 

 senting the various phases of electrical variation is 

 entirely comparable to the mechanical registration of 

 a movement by means of a needle which traces its 

 record on a moving strip of paper. This kind of 

 separation would be applicable in a great number 

 of cases if it did not require a special and rather 

 complicated apparatus, namely, that of a movable 

 slide and a clockwork motor. But with an ordinary 

 apparatus a similar separation can be obtained by 

 imparting an onward movement to the image itself 

 while the photographic plate remains in position. In 

 order to effect this, a rotatory movement round its 

 own axis must be communicated to the apparatus 

 itself between the periods of exposure. The principal 

 optical axis of the objective is thus displaced in a 

 horizontal plane ; and the image of a man standing 

 in front of the dark screen will consequently be 

 displaced in a corresponding direction on the plate 

 itself. If this man executes certain movements in 

 the same spot, thus constituting a variety of attitudes, 

 or if he advances excessively slowly, his successive 

 attitudes, instead of being confused and superimposed, 

 will constitute a series of disconnected images, ranged 

 side by side, as if he were moving at a moderate 

 pace in a horizontal direction in front of the dark 

 screen. 



But it may be urged that the conception of space 



