74 MOVEMENT 



black curtains to the smallest dimensions, thus augment- 

 ing the darkness. 



The majority of experiments do not require such 

 large backgrounds, and can be carried out quite easily 

 under the best conditions. 



A square box of 0*50 metre side, lined inside with 

 black velvet, makes an excellent background, especially 

 when the opening in the box is limited in size. In this 

 way photographs can be taken of the movements of 

 small animals, and, generally speaking, of all small 

 objects. The photographs in Chapter II. were taken 

 in front of a box of this kind. They show the figures 

 described in space by a white thread moving in all 

 three directions. 



Dark Background for photographing Objects in 

 Water. — For the study of the locomotion of fish, and 

 of other movements taking place under water, the 

 objects to be photographed must be themselves brightly 

 illuminated, while their surroundings are in total dark- 

 ness. For this purpose a rectangular tank with glass 

 sides is placed in front of a dark background; the 

 bottom of the tank is similarly made of glass, and 

 through it the sunlight is allowed to enter after being 

 reflected from a mirror set at an angle on the ground. 

 This arrangement is shown in Fig. 50. Here it will 

 be noticed that the glass tank forms part of an elliptical 

 canal, so that the animals can move round and round, 

 like horses at a circus. The canal is three-parts filled 

 with water, and the transparent part is illuminated by 

 means of an inclined mirror, which receives the sun's 

 rays direct, The canal, mounted on a high table, is 

 furnished with handles, so that it can be easily moved 

 from place to place. It is set out in front of an open 

 window, through which the sunlight can fall upon the 

 inclined mirror. The apparatus is variously disposed 

 according to the time of day, and the inclination of the 



