RADIATION AND TKMPKRATURE MEASURBM^NTS. 2$ 



brought about with the same efficiency as regards the effect upon the retina. 

 This appears very interesting viewed from the standpoint of the needs of 

 the human eye. But how do we know that the insect's eye requires Ught 

 fulfilUng our specifications; and if it does, why not say that the color of the 

 Hght varies for aesthetic reasons, rather than the more plausible one that it 

 serves as a mark of distinction between the various species ? 



IX. RADIATION AND TEMPERATURE MEASUREMENTS. 



The more recent attempts of Langley* and his assistants to measure the 

 radiation from the Cuban firefly show that if there are radiations lying 

 beyond the visible spectrum their heating value is immeasurable. The 

 earlier measurements of Duboisf seemed to indicate the presence of infra-red 

 radiation, but in view of the greater radiometric sensitivity now attainable 

 and the greater precautions taken to exclude extraneous radiations, the older 

 measurements must be assigned but little weight. The measurements of 

 Dubois indicate a very considerable amount of radiation (the galvanometer 

 deflection being i.8°) from the luminous organs of the Cuban firefly. The 

 dark parts of the body of the insect emitted radiation causing a deflection 

 of 0.95°. It is difficult to understand how, with the facilities now available, 

 these radiations, if present, can escape detection. 



The present measurements were first undertaken on samples of the Cuban 

 firefly {Pyrophorus noctilucus) . Out of the two dozen in the original ship- 

 ment, only one survived in transit, and this one was too weak for radiation 

 measurements. Thinking that the lack of intensity might be compensated 

 by using a larger radiating area, measurements were undertaken on the 

 luminous organs of the local firefly, when stimulated so as to emit a rich 

 glow. As mentioned elsewhere, a slight pressure on the luminous segments 

 will cause a strong glow. A vacuum thermopile, exhausted by means of 

 liquid air and charcoal (after preliminary evacuation with an oil pump) was 

 used as a radiometer. The sensitive parts consisted of a fine platinum wire 

 o. I mm. in thickness, and a strip of bismuth rolled to 0.02 mm. thickness, 

 6 mm. long, and of the shape indicated in Fig. 13, -S. The platinum wire was 

 used because of the ease with which it could be "tinned" and soldered to the 

 bismuth with Wood's alloy. The latter ordinarily makes a poor high- 

 resistance connection, but this is easily remedied by coating the wire (plati- 

 num or iron) with solder. The wide circular area, constituting the receiver, 

 was blackened with a mixture of lamp-black and platinum-black. The out- 

 side of the glass tube was covered with tin-foil, except openings on opposite 

 sides of sufficient size to admit radiation on the sensitive junction and for 

 the purpose of adjustment. This thermopile was entirely free from the lag 

 usually found in such instruments. The heating attained its maximum in 

 less than 2 seconds, which was the time required for the galvanometer to 

 reach the maximum deflection. 



It will be noticed that in certain work, for example where the insect was 

 near the instrument, radiation fell on both junctions, and hence not operat- 

 ing at its maximum efficiency. On actual trial it was found that it made 



*Annals Astrophys. Obs., 2, p. 5, 1902. 



jDubois, Bull. Soc. Zool. France, parts i, 2, and 3, 1886. 



