272 



Messrs. Langley and Very on the 



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 disappear from the inability of 

 the eye to follow a diminishing light. While we observe 

 from these first photometric measures that the insect's spec- 

 trum has undoubtedly a decided maximum in the green, we 

 are led to infer that this spectrum is very probably of the 

 nature of a broad band stretching 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 fines 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 ob- 

 serving telescope was a slit 1 millim. wide, subtending not 

 quite 9*5 (minutes of arc), which allowed light having a 

 range of wave-length of about O^'Ol 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 NicoFs prism. The angle through which the prism was 

 turned to produce equality having been noted, the values 

 deduced from the ordinary formula? (transmitted light 

 = I cos 2 a, the angle a being 90° when the light is diminished 

 as much as possible by crossing the planes of the Mcols 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 multiplied to produce equality with the 

 insect spectrum. 



Part of spectrum 



corresponding to 



centre of slit at 



focus of observing 



telescope. 



Blue- 

 green 

 very- 

 near 

 F. 



Green 



near 



b. 



Green 

 near 



E. 



Yellow 

 green. 



Citron. 



Yellow. 



Orange- 

 yellow 

 near 

 D. 



Orange. 



Wave-length 



Brightness 



049 

 0-02 



A* 

 0-51 



021 



V- 

 0-53 



0-34 



0-54 

 0-37 



056 

 0-24 



0-58 

 0-19 



0-59 

 0-17 



0-60 

 009 



* Alluded to but not fully described in the Am. Journ. Sci., August 1877. 



