80 MOVEMENT 



have acquired such skill that we can do no better than 

 borrow their methods. 



Generally speaking, objects should be directly 

 illuminated, but the dark background often makes 

 lateral illumination necessary. When this latter means 

 is employed, the contour of certain parts of the object 

 may be very well defined, but others may be too much 

 in the shade. This can be rectified by employing 

 reflectors properly inclined. To sum up, the problem 

 of illumination must be solved in a variety of ways, 

 but it is chiefly of importance in those rarer cases, in 

 which artistic effects are the chief aim. 



Disposition and Preparation of the Dark Field. — The 

 breadth which one must give to the background 

 depends upon the extent of the movement, the various 

 phases of which we want to follow. The opening must 

 correspond to the amplitude of the movement in such 

 a way that the least possible amount of light may 

 enter the box. In the same plane as that on which 

 the movement is to take place, a metre scale must be 

 fixed, and, if there is room, the chronometric dial 

 also. 



Lastly, the photographic apparatus must be placed 

 just far enough off for the sensitized surface to 

 correspond to the limits of the background. But to 

 regulate this, the limits of the background must be 

 visible on the ground-glass plate, and so they should 

 be indicated by placing on them bright-coloured strips, 

 or other striking objects. 



Choice of the Objective. — When the observed move- 

 ment is strictly confined to one plane, any sort of 

 objective can be employed, but it must be placed at 

 such a distance that the image on the ground-glass 

 plate assumes the proportions required. In this case 

 it is better to use an objective of short focal length, 

 since it admits a larger quantity of light. Under 



