12 



THE CHEAPEST FORM OB" LIGHT. 



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 fol- 

 lowed through O'^IS with the thoracic light, while with that of the same 

 character, but double the brightness, it was followed only through a verj' 

 little more, or 0'^-20, and while at tlie same time that of a but slightly 

 brighter artificial flame was followed through nearl}'' double or 0'^-38, it 

 seems probable that the insect's light actually ceases near the given 

 limits, and does not merely disappear from the inability of the eye to 

 follow a diminishing light. While we observe from these first ])hoto- 

 metric measures that the insect's spectrum has undoubtedly a decided 

 maximum in the green, we are led to infer that this spectrum is ver}' 

 probably" of the nature of a broad band sti'etching from beyond F to 

 near C where it terminates, and this very important inference we shall see 

 confirmed later by other and more exact measures. 



August 5. Comparison of relative brightness in different parts of 

 spectrum of abdominal light with that from a student's lamp. 



A spectrometer supplied with means* for bringing into the same field 

 the spectra of two different lights, formed by a Rutherfurd grating of 

 17296 lines to the inch (instead of the prism) was employed for this 

 purpose. The upper half of the slit received the insect's light, the 

 lower half a beam from the brightest part of the Argand flame, which 

 had passed through two Nicol's prisms, one of which was attached to a 

 divided circle. The two spectra were then seen in the same field with 

 their edges in exact juxtaposition. In the field of the observing tele- 

 scope was a slit 1"" wide, subtending not quite 9-5 (minutes of arc), 

 which allowed light having a range of wave-length of about 0'*01 to pass. 

 The spectrum of the lamp-light was brighter in every part of the field 

 though in unequal degrees till it was diminished by turning the Nicol's 

 prism. The angle through which the prism was turned to produce 

 equality having been noted, the values deduced from the ordinary for- 

 mula (transmitted light = I cos^ a, the angle a being 90° when the light 

 is diminished as much as possible by crossing the planes of the Nicols 

 at right angles) are as follows, where the fractions are those by which 

 the brightness of the lamp spectrum at the various points is to be multi- 

 plied to produce equality with the insect spectrum. 



*Alluded to but not fully described in the American Journal of Science, August, 



1877. 



