LUMINESCENCE AND INCANDESCENCE 31 



bluish fluorescence of quinine sulphate is really an emis- 

 sion rather than a reflection of light. But a test tube of 

 quinine sulphate solution held in the ultra-violet region of 

 a spectrum will glow with a pale blue light, although it is 

 not illuminated with any rays that are visible to our eyes. 

 Concerning this, Stokes, to whom the word fluorescence 

 and much of our knowledge of the subject is due, says, 

 ' * It was certainly a curious sight to see the tube ' ' (contain- 

 ing quinine sulphate solution) "instantaneously lighted 

 up when plunged into the invisible rays; it was literally 

 'darkness visible.' " Quinine sulphate absorbs the ultra- 

 violet converting these rays into visible blue ones. Its 

 spectrum is a short continuous one. Most fluorescent sub- 

 stances convert short into longer wave-lengths (Stokes' 

 Law), but some may cause the reverse change. 



A substance, fluorescent in solution, has been found in 

 a few luminous animals, notably in several species of 

 fireflies and also in a non-luminous beetle. It is called 

 pyrophorine or luciferesceine. Dubois (1886) has 

 ascribed to pyrophorine the power of absorbing invisible 

 rays and transforming them into visible ones, thus increas- 

 ing the animal's light. That this is not the case has been 

 shown by the work of Coblentz (1909). He photographed 

 the spectrum of the firefly's light and the fluorescent spec- 

 trum of luciferesceine. The latter is almost complemen- 

 tary to the former (see Fig. 4) and no trace of the fluores- 

 cent spectrum appears in the spectrum of the light of the 

 firefly. McDermott (1911 a) has studied the properties of 

 luciferesceine and regards it merely as an incidental 

 material found in many animals of the Lmnpyridce (in 

 some non-luminous forms) and having no connection with 



