228 History of Luminescence 



one but received little attention for over two decades, not until 

 Eduard Pfliiger's extensive papers on respiration and phosphorescent 

 organisms appeared in 1875. 



THE NATURE OF ANIMAL LIGHT 



Just as the middle of the nineteenth century was characterized by 

 a rapid accumulation of knowledge concerning all types of inorganic 

 luminescence, so the production of light by living things was sub- 

 ject to investigation of all kinds— its physical characteristics, the 

 source of light in luminous animals, i. e., the histology of luminous 

 regions, and somewhat later the chemistry of the process. Details 

 of these findings must be left for Chapter XVI on bioluminescence. 

 It will suffice to say that the first adequate studies on the spectrum 

 of animal light were made by Dr. Lehman (1862) and by J. Schnauss 

 (1862) , using the glowworm, and that light intensity and heat 

 studies were initiated by R. Dubois in 1886, using the elaterid beetle, 

 Pyrophorus. Many later workers established that animal light was 

 unpolarized as produced, and no different from any other kind of 

 light, except in spectral composition. The spectrum is always a short 

 band in various regions of the visible, either red, yellow, green, or 

 blue, depending on the animal. It is quite similar to the spectra 

 of phosphors and also to the spectrum of lophin. Lophin is an 

 organic compound which can be oxidized in solution with the emis- 

 sion of light, a discovery of B. Radziszewski (1877) , who also studied 

 its chemiluminescent spectrum in 1880 (see Chapter XIII on chemi- 

 luminescence) . Important biochemical studies were begun by R. 

 Dubois, whose work is considered in a subsequent section. 



It was also realized during the mid-century that bioluminescence 

 might result either from material secreted to the outside of the ani- 

 mal (extracellular luminescence) or the light might arise within 

 cells themselves (intracellular luminescence) . The first studies on 

 the fine structure of a complicated luminous organ, such as that of 

 the glowworm, were made by R. A. von Kolliker (1817-1905) and 

 by F. Leydig (1821-1905), both in 1857, to be followed by Max 

 Schultze (1825-1874) in 1863, and a host of later workers. 



The physiological aspects of light production and the relation 

 between luminescence and excitation became clarified after the 

 middle of the century. It was proved that special nerves existed, 

 whose stimulation carried nerve impulses to luminous cells but the 

 details of all these discoveries must be sought in Chapter XVI. 



