NORMAL, AND ABNORMAL MOVEMENTS. 105 



rapid movements. Therefore in a movement relatively so slow 

 as the walk they can be practically discarded. 



Another cause for slight loss of accuracy arises from the fact 

 that the person walking cannot be in exact apposition with the 

 lateral background which is used as a scale. This error is of course 

 lessened by the use of lenses of long focus, — l.e, in the lateral bat- 

 tery of cameras, which was placed upwards of fifty feet from the 

 track. Being by this means so much diminished, it can also be 

 discarded. A third source of inaccuracy is due to the fact that 

 the various phases of any movement photographed cannot be, ex- 

 cept by improbable accident, directly in front of the centre of 

 each corresponding camera. Therefore each individual phase, in- 

 stead of being photographed from a position exactly at right angles 

 to the background, is doubtless in the majority of instances photo- 

 graphed at an angle varying slightly from the right angle. Fur- 

 thermore, for the various phases, this angle is a variable and in- 

 determinate quantity. However, it must again be insisted that 

 for the majority of movements, especially slow movements, such 

 as walking, this error is also small, though it is obviously greater 

 than the second error just mentioned. 



Taken all in all, while the serial method gives slightly less 

 accurate results regarding the rise and fall and onward move- 

 ment of a limb, it more than compensates for this loss — which 

 in itself is slight — by enabling us to determine the amount and 

 direction of the lateral sway. Furthermore, apart from the mere 

 determination of trajectories, two photographs of any one phase 

 of movement taken at right angles give us an opportunity for the 

 study of the action of a part which a wheel-photograph can in no 

 way furnish. 



Having considered the various sources of error, let us take up 

 the study of the walk in Plate 1. The model was a young man 

 of medium height and was photographed while taking a long 

 step. 



By means of a transparent scale identical with that of the back- 

 ground, the vertical and forward movements of various points of 

 the body were readily studied. The lateral sway was determined 

 by means of a transparent scale based upon the broad divisions of 

 the background at the end of the track, — that is, the distance 

 between the heavy white lines, which is equivalent to thirty centi- 



