DESCRIPTION OF THE APPARATUS 71 



fixed plates demand that the objects, of which the 

 movements are to be studied, should be the only ones 

 to appear on the sensitized plate, and that the back- 

 ground should not throw a single ray of light into 

 the apparatus. 



A black velvet curtain may be used for this purpose, 

 provided that the sun does not shine directly upon it, 

 for all substances, however dark their colour, reflect a 

 certain amount of light when strongly illuminated. 



Chevreul pointed out that the only means of obtain- 

 ing absolute darkness was to blacken the inside of a 

 box, and make a hole in one of its sides. By the side 

 of this dark hole all black material illuminated by the 

 sun appears to be coloured. The nearest approach we 

 have been able to make to these ideal conditions of 

 Chevreul was by constructing a dark and capacious 

 shed (Fig. 49) at the Physiological Station,* the in- 

 terior of which has been painted black, and by hanging 

 a black velvet curtain at the back. The opening of the 

 shed is eleven metres long by four in height. This 

 opening is so situated that the sun cannot penetrate 

 into the interior. 



* This establishment, by permission of General Assembly and the 

 Municipal Council of Paris, was set up in Princes Park (Park des 

 Princes). Here it is possible to carry out certain researches which 

 would be impracticable in laboratories of the ordinary kind. Such a 

 field for research exists as yet nowhere else. There is a long circular 

 track, perfectly horizontal, and five hundred metres in circumference; 

 on this the ordinary paces of men and large animals can be studied. 

 By means of a dark background, it is possible to apply chrono photo- 

 graphy on fixed plates to the analysis of long-continued movements. 

 A background, uniformly illuminated, and of even surface, offers 

 facilities for chronophotography on moving films. Registering dyna- 

 mometers, spirometers, pedometers, and various apparatus for the 

 measurement of objects under observation are devoted to the study 

 of human locomotion. In addition, pneumographs, sphygrnographs, 

 and cardiographs enable the investigator to study the effect of athletic 

 exercises on the functions of organic life, ami to follow step by step 

 the improvement under training. Finally, there is an enclosure, 

 where various kinds of animals can be reared in liberty, and where 

 their normal and modified locomotion can be studied at pleasure. 



