SPACE 19 



the exact dimensions thus indicated. To introduce 

 such a scale, into the picture, a rule with very distinct 

 divisions must be placed by the side of the object 

 which is being photographed. When an artist wishes 

 to represent in relief an object of three dimensions, 

 he must obey the laws of perspective, and take into 

 consideration the manner in which the light falls on 

 the object, at the moment of drawing. But the fitful 

 changes of light, during the various times of day, offer 

 innumerable difficulties. 



Photography, however, gives an instantaneous picture 

 of the most diverse objects, and that, too, with the 

 prevailing conditions of light, and all in correct 

 perspective. The appearance of natural objects, as 

 seen by looking with one eye only, is thus repro- 

 duced by photography. If it is required to get 

 the effect of relief, such as is obtained bv looking 

 with both eyes, recourse must be had to stereoscopic 

 pictures. 



Photography traces ths Various Positions in Space 

 occupied by a Moving Body. — When an object changes 

 its position, it is often necessary to notify the posi- 

 tions in space which it successively occupies. In the 

 first place, we must be quite sure that our eyes have 

 boen able to follow the various phases of movement, 

 and that our memory has been able to retain the 

 details— conditions rarely fulfilled — before we have 

 recourse to drawing, as a means of representing the 

 trajectory described. The diagram traced will be 

 more or less complicated according as we express the 

 movement by a point, a line, a plane superficies, or a 

 solid — in short, according as we represent a movement 

 executed in one or more directions. 



When it is only a question of the movement of a 

 point, in certain cases the difficulty may be met by 

 making the moving point itself trace the path which 



