Langley and Very — Cheapest Form of Light. 10 7 



light. While we observe from these first photometric meas- 

 ures that the insect's spectrum 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 im- 

 portant 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 observ- 

 ing telescope was a slit l mm 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 formula (transmitted light =1 cos 2 a, the angle a being 

 90° when the light is diminished as much as possible by cross- 

 ing the planes of the ISTicols 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 pro- 

 duce equality with the insect spectrum. 



Fart of spectrum 



Blue 

















corresponding to 



green 













Orange 





center of slit at 



very- 



Green 



Green 



Yellow 







yellow 





focus of observing 



near 



near 



near 



green. 



Citron. 



Yellow. 



near 



Orange. 



telescope. 



F 



b 



E 









D 







/" 



/" 



/" 



/" 



/" 



M 



/" 



/" 



Wave-length, 



0-49 



0-51 



0'53 



0-54 



0-56 



0-58 



0-59 



0-60 



Brightness, 



0-02 



0-21 



0-34 



0-37 



0-24 



0-19 



0-17 



0-09 



Owing to the motion of the insect and the varying brilliancy 

 of the light emitted, these figures (each of which is the result 

 of the mean of several trials including at least two measures) 

 still leave much to be desired. The supply of the insects 

 which had been procured and maintained alive with difficulty, 

 however, did not allow of the experiments being further pro- 

 longed, nor of the securing a direct comparison with the solar 



* Alluded to but not fully described in this Journal, August, 1877. 



