distinguish between _Photinus_ and _Photuris_ in nature, merely by the reddish tint of the light of the former. These photographic records show conclusively that the color of the light of luminous animals is not a subjective phenomenon due to the Purkinje effect and the low intensity of the light, but is real, an actual difference in spectral composition of the light emitted. Neither is it due, at least in the fireflies examined, to the existence of color screens which absorb certain rays, allowing only those of a definite color to pass. The spectra of forms thus far investigated are reproduced in Fig. 9 and recorded in Table 5. It will be noted that they vary considerably in position but are all of the same type. The spectrum of _Cypridina hilgendorfii_ is the longest thus far investigated (λ = .610µ to λ = .415µ), extending well into the blue, and the light of this form is very blue in appearance. [Illustration: FIG. 7.--Spectra of carbon glow lamp, A, firefly (_Photinus pyralis_); B, and helium vacuum tube, C (_after Ives and Coblentz_).] [Illustration: FIG. 8.--Spectra of helium vacuum tube (1); carbon glow lamp (2); the firefly, _Photinus pyralis_ (3); and the firefly _Photuris pennsylvanica_ (4) (_after Coblentz_).] [Illustration: FIG. 9.--Spectra of various luminous animals (_after McDermott_). 1. Portion of the visible solar (grating) spectrum showing Fraunhofer lines. 2. _Pyrophorus noctilucus_ (Langley and Very.) 3. _Lampyris noctiluca_ (Conroy). 4. _Photinus pyralis_ (Ives and Coblentz). 5. _Photinus consanguineus_ (Coblentz). 6. _Photuris pennsylvanica_ (Coblentz). 7. _Phengodes laticollis_ (McDermott). 8. _Bacterium phosphoreum, B. phosphorescens or Bacillus photogenus_ (Molish). 9. _Photobacterium indicum_ (Barnard). 10. _Mycelium X_ (Molish). 11. _Luminous bacteria_ (Förster). 12. _Agaricus sp._? (Ludwig). 13. Fluorescent spectrum of luciferesceine of _Photinus pyralis_ (Coblentz). Only the extreme ends of the bands are shown and no attempt is made to indicate the relative density of different portions of the spectra.] TABLE 5.--_Limits of Spectra of Various Luminous Organisms_