WORK OF THE PHYSIOLOGICAL STATION AT PARIS. 403 



anatomists, I have been able to collect a portion of tbe specimens nec- 

 essary for the stndy of certain common species of mammals. 



I have also been able to collect, in the physiological department, 

 chrc no-photographs and sketches of the movement of the same species. 



Plates L and LI show a series of thirty-six photographs of a dog; 

 taken while making a step. For such a slow gait the mimber of pictures 

 is greater than necessary, for a more rapid one all would be required. In 

 the present case it will be sufiicicnt to compare every four pictures to 

 follow the phases of the movement. As the chronometric dial is used to 

 measure the intervals of time between two successive attitudes, so 

 the space passed over by the animal is measured by a mark fixed in the 

 ground. This mark consists of a small white rod set up on the course. 

 At the commencement of the experiment the dog is behind the rod, at 

 the end he has passed it. The actual distance traversed is read oft" 

 upon a metric scale. 



Plate LII shows a sheep walking. The number of photographs is 

 reduced to nine, and the time between the tirst and last, as shown by 

 the chronometric dial, is twenty -five sixtieths of a second. The pro- 

 gression of the animal is estimated by his approach to the chronometer. 



To obtain these pictures a dark background is used. This is not nec- 

 essary when a movable film is employed; then a light background will 

 answer, as shown in ])late LIII, which represents a galloping horse. As 

 this was quite a long series, it was not possible within the space of a 

 column of text to show the entire set of movements; about half are 

 shown from the moment when the horse leaves the ground to that 

 when the two diagonally opposite feet are on the ground. 



It is sometimes desirable to take pictures of an animal from different 

 aspects; chrono photography upon a movable plate is easily adapted 

 to such work. It is possible to follow in any gait of an animal the 

 series of movements of his limbs. 



But the dimensions of the pictures obtained by printing from the 

 original negatives are too small to be directly utilized. Arrangements 

 must be made to more easily examine and measure the displacements 

 made by the different portions of the same limb during the step of any 

 gait. For this we proceed as follows: 



We commence by enlarging- by means of a projection apparatus each 

 of the little images which we wish to compare. Plate LI V shows one of 

 these enlargements. It is limited to five diameters, so as to accommo- 

 date the page, but it is better to nearly double those dimensions. 



If we wish, for example, to study the movement of one of the hind 

 legs we trace the contours of that leg on a sheet of transparent ])aper, 

 using as guides in applying this paper to the picture the line of the 

 ground and a fixed point upon that line. Having- traced the first 

 image of the series we proceed to the second, placing the transparent 

 paper exactly over the guides, and thus obtain (fig.l ) the series of suc- 

 cessive positions occupied by the hind leg of a horse while walking, 

 from the first raising of the foot to its final return to the ground. 



