AERIAL LOCOMOTION 



253 



the wings. So we gradu- 

 ally reduced the diameter 

 of the fenestrations by 

 drawing the metal cur- 

 tains which regulated the 

 size of the openings. 



When the openings in 

 the two diaphragms were 

 only I'D millimetre in 

 breadth, the duration of 

 the exposure was reduced 

 to 25 000 °f a second, and 

 in this case the photo- 

 graphs obtained never 

 failed to be absolutely 

 distinct. 



An insect flying against 

 the pane of glass must 

 occupy a considerable 

 amount of space in an 

 antero - posterior direc- 

 tion, and hence if all 

 portions of the body are 

 to be well defined, the 

 object-glass must be one 

 of considerable focal 

 length. Xow, as a mat- 

 ter of fact, the extreme 

 narrowness of the fenes- 

 tration through which 

 the light passes, and the 

 corresponding smallness 

 of the openings of the 

 diaphragms give a focal 

 length of rather more 

 than 2 centimetres. 

 18 



