RADIATION AND TEMPERATURE MEASUREMENTS. 29 
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 light 
fulfilling our specifications; and if it does, why not say that the color of the 
light varies for esthetic 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 Duboist 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 1.8°) 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°. Itis 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 aradiometer. The sensitive parts consisted of a fine platinum wire 
0.1 mm. in thickness, and a strip of bismuth rolled to 0.02 mm. thickness, 
6 mm. long, and of the shapeindicated in Fig.13,B. 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 suchinstruments. 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. 
tDubois, Bull. Soc. Zool. France, parts 1, 2, and 3, 1886. 
