THE CHEAPEST FORM OF LIGHT. 9 



process (presumably discoverable ])ut still unknown) gives light without 

 invisible heat. 



These observations are : 1. Photometric. 2. lliermal. 



Part 1. — Photomktrio Observations. 



The first impression on viewing the light of the Pi/roj^horus noctUucus 

 through a spectroscope is that it consists essentially of a broad band in 

 the green and yellow, while with })recaution we see this extending into 

 and beyond the borders of the blue and orange, ])ut not very greatl}^ 

 farther, and these have been taken by previous observers as its absolute 

 limits. No one appears to have experimentally and distinctl}' answered 

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

 to be seen ? " nor has it been proven 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. 



Tt is impossible to increase the intrinsic brillianc}^ 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 t)ther hand quite possible to make the 

 light of the sun no brighter than that of the insect, and this would ap- 

 pear to 1)6 the first step in obtaining 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'' photo- 

 metric device ; and having taken this sunlight as nearly equal as possible 

 to that of the insect, though certainly not greater, to let this determined 

 quantit}^ 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. 



The actual doing this is not so easy as it might appear, owing to ex- 

 ]ierimental difficulties connected with the insect, a part of Avhich arises 

 from the fact that its light is not only fitful but unequal, being of very 

 varying intensities when not wholl}^ intermittent. 



The simplest way in which the experiment can be performed is per- 

 ha})s the following: 



The insect is placed immediately in front of the slit of a spectroscope, 

 so that the light of its thoracic or abdominal ])ortion falls upon the slit. 

 This forms a narrow spectrum which should be brought into the lower 

 or ujiper half of the field, the insect })eing attached to the s})ectroscoi)ic 

 a])])aratus in a position as nearly fixed as possible. The Spectroscoi)e 

 is now placed with the axis of its collimator in the line of a ray of sun- 

 light cast from a heliostat without. In the path of this ray is a screen 



