Cheapest Form of Light, 263 



Riley, and to Professor L. A. Howard, to whose knowledge 

 and kind care I am doubly indebted. 



After a preliminary spectral examination in Washington, I 

 found it more convenient to continue the research at the 

 Allegheny Observatory by means of the very special appa- 

 ratus supplied by the liberality of the late William Thaw, of 

 Pittsburgh, for researches in the lunar heat-spectrum*. Pho- 

 tometric measurements throughout the spectrum of the insect's 

 light were also made. 



I have indicated the steps of the investigation ; but the 

 experiments have been so largely and so intelligently made by 

 Mr. F. W. Very, that it is just to consider him as an associate 

 rather than an assistant in the researches. I shall accordingly 

 in what follows not discriminate between what each has 

 contributed. 



Historical Notes. 



We make no attempt to give any bibliography of the 

 subject, and these notes are confined to what seems import ant 

 in the history of the physical side of it. 



Nathaniel Hulme\. — Exp. 6. A dead shining glow-worm 

 was put upon water contained in a wide-mouthed phial, at the 

 temperature of 58°. The phial was then sunk in boiling-hot 

 water ; and as the heat communicated itself to the contents 

 of the phial, the light of the glow-worm became much more 

 vivid. 



Exp. 7. Another lucid dead glow-worm was put into warm 

 water at 114°, to see if that degree of heat would extinguish 

 the light ; but, on the contrary, its glowing property was 

 augmented. All the water was then poured off, yet the insect 

 continued to shine for some length of time. 



Exp. 8. Two living glow-worms were put into a one-ounce 

 phial, with a glass stopple ; and though they were perfectly 

 dark at the time, yet if the phial were briskly rubbed with a 

 silken or linen handkerchief till it became pretty warm, it 

 seldom failed to make them display their light very finely. 

 This experiment was very frequently repeated. It had 

 the same illuminating effect upon the light of a dead glow- 

 worm. 



Exp. 9. The complete influence of 212 degrees of heat was 

 now applied to the fight of a glow-worm by pouring upon 

 one when dead, but in a luminous state, some boiling water. 



* Described in the Memoirs of the National Academy, vol. iv. part 2, 

 p. 112. 



t Phil. Trans. Roy. Soc. Lond. vol. xc. pp. 180-181 (1800). 



