Cheapest Form of Light. 271 



20 millim. long, the parallel wires being distinctly visible in 

 the indigo at a setting of 45° 25', corresponding to a wave- 

 length of 0^*468, and in the red at 43° 53', corresponding to 

 0' i- 640. The spectrum then was visible from a little beyond 

 F to near C, or through a range of ,J "172. As might have 

 been anticipated from the greenish colour 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 fig. 3, 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 

 thoracic 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 centirn., 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. 



o , P 



Wires seen in indigo .... 4 529 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 centim. high, at point one third down 

 from top, just within inner and slightly darker cone, seen 

 through hole 2*5 millim. in diameter). Under these circum- 

 stances 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 0^*18 with the thoracic light, while 

 with that of the same character but double the brightness it 

 w r as followed only through a very little more, or K 20, and 



* In the normal heat-spectrum the maximum has a wave-length fl - 57. 



U2 



