22 MOVEMENT 



movement of the point of a wing cannot be expressed 

 comprehensively in the form of a plane diagram, since 

 the movement of the wing at the shoulder-joint takes 

 place in three directions. The photographic diagram 

 only gives one projection of this trajectory. It is thus 

 incapable of expressing the real course of flight taken. 

 If a movement takes place in three directions, recourse 

 must be had to a more complicated arrangement, and 

 a stereoscopic trajectory obtained. Let us take the case 

 of a man walking away from us, and concentrate our 

 attention on a particular point of his body. This point 

 is elevated and depressed as the man's foot rises and 

 falls. Further, it is affected by the side-to-side swing, 

 and according to the direction in which he walks. 

 The pedestrian must be completely dressed in black, 

 and a bright metal button fastened to a part of his 

 body. The man is then made to walk in front of a 

 dark background, and a stereoscopic photographic 

 camera with two lenses is focussed on the spot. Both 

 of these object-glasses, acting precisely as the single 

 one in the preceding experiment, produce two images 

 of the luminous point. 



Fig. 14 was obtained in this manner ; it shows two 

 images of the same trajectory taken from two different 

 points of view. Examined with the stereoscope, these 

 images stand out in bold relief.* 



* Since a considerable number of people are able to see figures of 

 this sort standing out in relief without having recourse to a stereo- 

 scope, we have published the above figure, find certain others will 

 be found further on. To obtain the full effect of relief without :i 

 stereoscope, we must concentrate our vision on a distant point, and 

 then interpose the object between our eyes and the distant point. The 

 page of the book will then be seen double, and consequently the tra- 

 jectory will upper as four separate images. If the book is then very 

 gently moved, and the direction of the eyes slightly alten d, the two 

 internal images finally become exactly superimposed. The eye is 

 then accommodated for distinct vision of the central image, which 

 stands out in relief between the two outlying images. 



With a little practice one can easily dispense with the stereoscope 

 for examining figures of this sort, 



