HO BIOLOGICAL LECTURES. 



of a broad band in the green and yellow, while with precaution 

 we see this extending into and beyond the borders of the blue 

 and orange, but not very greatly farther, and these have been 

 taken by previous observers as its absolute limits. No one 

 appears to have experimentally and distinctly answered the 

 question, " Would the light not extend farther were it bright 

 enough to be seen ? " nor has it been proved as clearly as 

 might be desired that the result depends on the quality rather 

 than the quantity of the light, or given conclusive evidence 

 that if the light of the insect were as bright as that of the 

 sun, it would not extend equally far on either side of the 

 spectrum. 



" It is impossible to increase the intrinsic brilliancy by any 

 optical device, but if it be impossible to make the light of the 

 insect as bright as that of the sun, it is, on the other hand, quite 

 possible to make the light of the sun no brighter than that of 

 the insect, and this would appear to be the first step in obtain- 

 ing a definite proof that the apparently narrow limits of the 

 insect's spectrum are due to the intrinsic quality of the light, 

 and not to its feeble intensity. The only conclusive method of 

 determining this would appear to be to balance the light from 

 the insect with that of a definite portion of -sunlight by any 

 ordinary photometric device ; and having taken this sunlight 

 as nearly equal as possible to that of the insect, though certainly 

 not greater, to let this determined quantity fall on the slit of 

 a spectroscope at the same time with the light from the insect, 

 two spectra being formed one over the other in the same field 

 and at the same time " (pp. 103, 104). 



After detailing a number of experiments, the authors state 

 that " when spectra are formed from two equal lights, one from 

 the sun, the other from the insect, the latter* s spectrum termi- 

 nates both at an upper and a lower limit, at which the solar 

 light is still conspicuous. The conclusion follows that the 

 insect spectrum is lacking in rays of red luminosity, and pre- 

 sumably in the infra-red rays, usually of relatively great heat, 

 or that it seems probable that we have here light witJiout heat, 

 other than that heat which the luminosity itself comprises and 

 which is but another name for the same energy" (p. 108). 



