Cheapest Form of Light. 



269 



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 conse- 

 quently 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 millim. by 

 1*5 millim. (see fig. 1). Their light is not so bright as the 

 abdominal light, but much steadier, and, like that, of a de- 

 cidedly greenish hue. One of these oval spots was placed 

 over the centre of a slit, open just enough to receive the light, 

 or about 1*5 millim. This slit was in the focus of a glass lens 

 of 8 centim. aperture and 82 centim. focus, which acted as a 

 collimator. The prism was a very large one of flint (faces 

 11*5 centim. high, 105 centim. 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 



Kir. 1. 



Ficr. 2. 



Pyroph&rus noctilucus. 



parallel wires. The whole was mounted on the spectrometer, 

 primarily designed for bolometric measures and fully described 

 PUL Mag. S. 5. Yol. 30. No. 184. Sept. 1890. U 



