RADIATION AND TEMPERATURE MEASUREMENTS. 31 



heat radiation. However, it is not considered a true endothermic phenome- 

 non caused by the absorption of heat by the photogenic organs, but is rather 

 a slow conduction of heat from the walls of the radiometer to the insect, 

 which is at a lower temperature. If the phenomenon is one of a true trans- 

 fer of radiant energy, then the galvanometer should have reached its maxi- 

 mum deflection in 4 seconds instead of 60 to 80 seconds, and it should have 

 remained at a maximum instead of slowly drifting back after the expiration 

 of 1.5 to 2 minutes. The crucial test would be to use a rock-salt window 

 which would permit the long- wave radiation from the thermo- junction to 

 pass directly to the insect, instead of impinging on the glass walls of the 

 radiometer. This cooling effect led to the thermo-couple explorations to be 

 described presently. 



The radiometric tests of the flash and the glow of both Photuris Pennsyl- 

 vania and Photinus pyralis extend over a period of 3 weeks, during which 

 time four distinct series of measurements were made ; each series took from 

 1 to 3 hours, during which time various experiments were tried, using red 

 absorption glass, etc. In the earlier measurements on the Photuris the 

 insect was attached to a thin wooden rod about 15 cm. long, held horizon- 

 tally and suspended over the thermo-junetion. The tests were made in a 

 darkened room and shields were provided so that no stray light could get 

 into the thermopile case. The flashes were as frequent as 3 times per 

 second, but in neither the flash nor the steady glow could any radiation be 

 detected with certainty other than the cooling effect (conduction of heat) 

 which will now be discussed. 



If it were not for the novelty of the phenomenon and the averseness with 

 which one views data of this type, after having been taught to search for a 

 heating effect rather than the opposite, it would be possible to urge that the 

 emission of light is accompanied by an absorption of heat. For example, a 

 strong, active Photuris, flashing 80 to 90 times per minute, caused a cooling 

 effect of 5 to 6 cm. deflection (galvanometer period 4 seconds, therefore very 

 steady and no " drift) ; and later on, when the flashes became less frequent, 

 in two distinct series of measurements the maximum (cooling) deflection 

 was in each case only 3.5 cm. Another specimen gave a (cooling) deflection 

 of 1.5 cm. and then began "flashing brighter" (as the records have it), when 

 the deflection increased to 3.1 cm. Still another specimen of Photuris, 

 "flashing 3 times per second," gave a maximum (cooling) deflection of 6 to 7 

 cm., and finally only 2.2 cm. deflection. The records show several similar 

 cases. Whether these measurements have any significance it is difficult to 

 say. The closer to the radiometer the greater was the cooling, but it is not 

 known whether the insect was accurately at the same distance from the walls 

 of the radiometer when flashing and when it became less active. 



If the insect generates only the radiations which are visible to the eye, the 

 heating value of these rays will be small. It is evident that but little infra- 

 red radiation can be generated, for so much would be absorbed that unless 

 it has some cooling device the temperature would rise to a high value and 

 the insect would be speedily desiccated. It is therefore possible to have the 

 radiation (heating) effects on the radiometer masked by the thermal con- 

 duction effects (cooling) if the temperature of the insect is less than that of 

 the thermopile. 



