JLangley and Very — Cheapest Form of Light. 105 



that under these conditions the sunlight is equal but not supe- 

 rior to that of the insect. 



The necessary condition of equality of the two lights from 

 which the spectra are to be formed, having thus been secured, 

 the grating is moved until the two spectra are brought into 

 the field. The result of this direct test is that the solar spec- 

 trum when intrinsically of the same brightness, or even when 

 clearly of less brightness than that of the insect extends some- 

 what further toward the red and distinctly further toward the 

 violet, the insect light being more intense than that of the sun 

 for equal lights in the green, but ending more abruptly on the 

 violet side. 



It may be added that when the insect's light grew brighter, 

 the increment appeared to be more in the blue end or as if the 

 average wave-length diminished, with the intensity, but there 

 was not opportunity to put this beyond doubt. 



Photometric observations in the prismatic spectrum were 

 made previously to the adoption of the arrangement above 

 detailed, the first being on July 1, 1889, using thoracic light. 

 The insect was mounted on an adjustable stand to which it 

 was attached loosely, so as to give it such freedom of motion as 

 is needed to ensure its emitting the light. It was 'consequently 

 necessary to re-adjust its position incessantly, and this necessity 

 constitutes a very obvious difficulty. The thoracic light spots 

 are two ovals, each about 2 mm by l-5 mm (see Plate III, fig. 1). 

 Their light is not so bright as the abdominal light, but much 

 steadier, and like that, of a decidedly greenish hue. One of 

 these oval spots was placed over the center of a slit, open just 

 enough to receive the light, or about l"5 mm . This slit was in 

 the focus of a glass lens of 8 cm aperture and 82 cm focus, 

 which acted as a collimator. The prism was a very large one 

 of flint (faces ll - 5 cm high, 10 - 5 cm wide), whose mounting 

 included an automatic minimum deviation attachment. The 

 observing lens was similar to the collimator, with a low-power 

 eye-piece in whose field was a pair of heavy vertical parallel 

 wires. The whole was mounted on the spectrometer, primarily 

 designed for bolometric measures and fully described else- 

 where.* The insect turned so as to show the abdominal light 

 is depicted in Plate III, fig. 2, the form of this latter organ on 

 the enlarged scale in Plate III, fig. 3. 



The observer waited for some time in a wholly darkened 

 room, and to the eye thus rendered sensitive, the visible spec- 

 trum, before magnification, was about 2 mm high and 20 mm long, 

 the parallel wires being distinctly visible in the indigo at a 

 setting of 45° 25', corresponding to a wave-length of O^GS, 

 and in the red at 43° 53', corresponding to 0-"-640. The spec- 



* See this Journal, March, 1883, p. 188. 



