THE CHEAPEST FORM OF LIGHT. 11 



and 82"" focus, which acted as a collimator. The prism was a very- 

 large one of flint (faces 11*5"" high, 10"5™ wide), whose mounting in- 

 cluded an automatic minimum deviation attachment. The observing 

 lens was similar to tlie collimator, with a low-power eye-piece in whose 

 field was a pair of heavy vertical parallel wires. The whole was mounted 

 on the si)ectrometer, primarily designed for bolometric measures and 

 fully described elsewhere.* The insect turned so as to show the abdom- 

 inal light is depicted in plate I, Fig. 2. 



The observer waited for some time in a wholly darkened room, and 

 to the eye thus rendered sensitive, the visible spectrum, before magnifi- 

 cation, was about 2°"" high and 20""° long, the parallel wires being dis- 

 tinctly visible in the indigo at a setting of 45° 25', corresponding to a 

 Avavedength of 0^-468, and in the red at 43° 53', corresponding to 0'^-640. 

 The spectrum then was visible from a little beyond F to near C, or 

 through a range of 0'^T72. As might have been anticipated from the 

 greenish color of the light, the maximum brilliancy was in the green 

 near E, or near wave-length O'*o3.t From this point the light fell away 

 on both sides more rapidly than in the solar spectrum. (See plate 

 II, A, B.) 



July 2. A comparison of the spectra of the thoracic and of the ab- 

 dominal light gave the latter upon the average about double the 

 intrinsic brightness of the former. This was only a crude estimate, but 

 more exact methods under the limited time for experiment would have 

 been useless, owing to the very fluctuating character of the light. In 

 continuation of the photometric measurements of the preceding day on 

 the thoracic light, this was compared with tliat from the flame of an 

 ordinary Bunsen burner at its greatest luminosity, whose area was 

 limited by a diaphragm to that of the size of the thoracic light. The 

 light from the base of this luminous flame (height of flame about o'o"'", 

 air shut off at base of burner) gave a continuous spectrum, which in 

 these first comparisons was alternated with that of the insect. The 

 spectra were judged to be equal in the blue and the red, but that of the 

 insect was much brighter in the green. Again, a spectrum being formed 

 from light taken midway between the base and point of the flame was 

 found to be everywhere too bright, but especially so in the red. 



July 3. Continuation of photometric measures, but with abdominal 

 light. (The abdominal luminous organ is shown in plate I, Fig. 2.) 



(Luminous flame 4™ high, at point one-third down from top. just within 

 inner and slightly darker cone, seen through hole 2-5"™ in diameter.) 



*Anierican Journal of Science, March, 1883, p. 188. 



t In the normal spectrum the maxinnun has a wave-length 0/^-57. 



