344 



KNOWLEDGE. 



September. 1910. 



of remarkable puritv. being comparable to that of a 

 violin. 



The optical s\'stem concentrating the beam of 



^ 

 * 



I'IGURE 5. 



A Drai;on Fly as seen coining towards the observer. 



light on to the objective consists of quartz lenses 

 which absorb far less of the chemicallv active ravs 

 than glass. The objective consists of a system 

 of quartz and calcite lenses, which hitter possess 

 the same properties in regard to absorption (sec 

 Figure 7). 



As the pictures obtained with the high speed 

 cinematograph on a bright background are neces- 

 sarily silhouettes, it is often difficult properly to 

 appreciate their perspective value. Figure 4 thus 

 shows a dragon fly in full flight which, according 

 as the observer sees the head of the insect turned 

 towards him, or from the opposite side, can be 

 interpreted in two different wa\'s. In order to do 

 away with this uncertaint\- the experimenter uses a 

 double or stereoscopic objective producing two \iews 

 simultaneously, as seen with the left and right 

 eye respectively, and which, taken together, gi\'e a 

 perfectly plastic impression. The spark gap has 

 obviously to be likewise duplicated so as to provide 

 for two parallel sources of light. 



As the film during an experiment should be 

 exposed to the light of the sparks i>nl\- during a 

 single rotation of the roll, the apparatus is provided 

 with an electrically-operated shutter, which is opened 

 rapidly at the proper moment, and closed automa- 

 tically again after the roll has performed a complete 

 revolution. This shutter consists of a brass plate 

 with two rectangular windows of the same dimensions 

 and distances as cinematograph pictures. These two 

 windows are closed previous to the experiment bv a 

 small shutter of very thin sheet steel, which is dis- 

 engaged at the proper moment bv a special electro- 

 magnet so as to open the two windows. At the end 

 of the experiment another shutter (also operated by 

 an electro-magnet) automatically closes the windows. 



The time is registered by means of an electro- 

 magnetic recorder controlled by a tuning fork of 

 fifty vibrations per second, the ends of which are 

 photographed along ^\ith the pictures. However, 

 in most cases, it will be sufficient to compare the 

 sound given out h\ the spark with that of a given 

 tuning fork in order to ascertain the speed of 

 working. 



The aj)paratus abo\-e described was mainly con- 

 structed for the purpose of photographing the flight 

 of insects. All the endeavours so far made to fix 

 this fugiti\'e motion on the photographic plate ha\-e 

 given only parth' satisfactory results, no experimenter 

 being able to record a successive series of instan- 

 taneous pictures. Moreover, the insect was mostly 

 kept captix'e. and therefore could not be investigated 

 under normal conditions. M. Lucien Bull has 

 therefore made a special point of keeping the insect 

 under test at full hbert\", confining its captivity to a 

 few seconds previous to the exposure. .Again, in order 

 to impart to its flight the proper direction towards the 

 photographic objecti\'e, he avails himself of the fact 

 that insects will nearl\- invariably fly towards the 

 light, and therefore arranges his ajiparatus close to 

 the window. The insect liberated on the opposite 

 side of the table will fly towards the window, and on 

 passing in front of the optical system is photographed 

 bv the apparatus. The motion recorded thus is that 

 of the first moments of flight which are the most 

 instructive, as their speed is comparativeh" low. 



The insect to be photographed is kept imprisoned 

 by a small electro-magnetic grip inserted in the circuit 

 of the electric shutter. (See Figure 5). As soon 

 as this circuit is closed, an electro-magnet will open 

 the grip, thus liberating the insect, which flies away 

 immediateh'. 



In the case of those insects which onl\- fl\' awa\' 



FlGliRE 5. 

 .A Fl\- held by the apparatns pre\-ious to being Ubcrated. 



after some time the experimenter uses a glass tube 

 with slanting ends (see Figure 8), closed by an 

 extremeh- light mica window which can be opened 



