106 Langley and Very — Cheapest Form of Light. 



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

 through a range of G^-172. 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 0" - 53.* From 

 this point the light fell away on both sides more rapidly than 

 in the solar spectrum. (See Plate IV, A, B.) 



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

 the abdominal 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 tho- 

 racic light, this was compared with that 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 3 - 5 cm , 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 ecpiial 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. (An outline of the abdominal luminous 

 organ is given in Plate III, fig. 3.) 



Wires seen in indigo 45° 29' 



0-463 



Abdominal light. 



red 43 47 



0-663 



Range 0<"-200. 



" " indigo 46 56 



0-390 



Range 0<"'382. 



" " red 43 21 



0-772 



Bunsen burner. 



(Luminous flame 4 cm high, at point one-third down from top, 

 just within inner and slightly darker cone, seen through hole 

 2 - 5 mm in diameter.) Under these circumstances the spectrum 

 of the insect's light was in the green a fair match for that of 

 the burner, elsewhere the latter was brighter but not very 

 greatly so. Since the insect's spectrum was followed through 

 O^-IS with the thoracic light while with that of the same char- 

 acter but double the brightness it was followed only through a 

 very little more, or O""20, and while at the same time that of a 

 but slightly brighter artificial flame was followed through 

 nearly double or 0<"-38, it seems probable that the insect's light 

 actually ceases near the given limits, and does not merely dis- 

 appear from the inability of the eye to follow a diminishing 



* In the normal spectrum the maximum has a wave-length QK*l. 



