AEE1AL LOCOMOTION 



251 



that is thrown on the insect very concentrated. Fig. 

 176 is a general scheme of the arrangement we 

 adopted. In the first place, it will be noticed that 

 there is a parallel beam of* light travelling from right 

 to left, and directed by a heliostat towards the principal 

 optical axis of the object-glass. This beam of light is 

 condensed by a lens* (c) behind which the insect 



Fig. 177.— Chronophotographic apparatus arranj 



insects. 



for studying the natural flight oi 



can be seen between the points of a forceps. The 

 condensed beam of light traverses the first lens of the 

 object-glass, and the rays are brought to a focus at 

 the circular diaphragms ; at the moment two fenestra- 

 tions coincide the rays can pass through and illuminate 

 the field of the movable film, in the middle of which 

 a silhouetted image of the insect stands out in bold 



* The focal length of this lens should be at least double that of 

 the objective. 



