As we have seen, difference in color of the light does not necessarily indicate difference in spectral composition because of the Purkinje effect. However, examination of the spectrum of various luminous forms has very clearly indicated that the different colors are really due to light rays of different wave-length and are not the result of any subjective phenomena. To facilitate comparison, spectral lines and colors are given in Table 4. The first adequate observations on the spectra of luminous animals were made by Pasteur (1864), who studied _Pyrophorus_ and found a continuous spectrum unbroken by light or dark bands. Lankester (1868) discovered a similar continuous spectrum in _Chætopterus insignis_ and placed its limits from line 5 to 10 on Sorby's Scale (about λ = 0.55µ to λ = 0.44µ). Young (1870) first recorded the limits of the firefly spectrum as a little above _C_ (λ = .6563µ) to _F_ (λ = .4861µ). Since then a number of luminous forms have been examined and all are found to give short continuous spectra (not crossed by light or dark bands or lines) lying in different color regions. Thus, Conroy (1882) examined the glowworm (_Lampyris noctiluca_) light and observed a band extending from λ = 0.518µ to λ = 0.656µ. Dubois (1886) states that the spectrum of _Pyrophorus noctilucus_, the West Indian "Cucullo," extends from slightly further than the Fraunhofer _B_ line to the _F_ line, while Langley and Very (1890), working on the same form, placed the limits at λ = 0.468µ to λ = 0.640µ. It consists, then, of a broad band chiefly in the green and yellow. But, "would the light not extend farther were it bright enough to be seen?... if the light of the insect were as bright as that of the sun would it not extend equally far on either side of the spectrum?" "It is impossible to increase the intrinsic brilliancy by any optical device, but if it be impossible to make the light of the insect as bright as that of the sun, it is on the other hand quite possible to make the light of the sun no brighter than that of the insect ..." Langley and Very investigated this question, forming a solar spectrum from sunlight of the same intensity as that of _Pyrophorus_ and a _Pyrophorus_ spectrum together in the same field of the spectroscope. The latter was very much shorter than the solar spectrum, showing that its length was not due to weakness of the red and blue rays but to their absence. Later Ives and Coblentz (1910) photographed the spectrum of a firefly (_Photinus pyralis_), together with that of a carbon glow lamp, on plates sensitive to all wave-lengths of visible rays under conditions which would have recorded all visible radiations given off. They found the spectrum to extend only from λ = 0.51µ to λ = 0.67µ (Fig. 7). Another species of firefly (_Photuris pennsylvanica_) was found by Coblentz (1912) to give a spectrum extending from λ = 0.51µ to λ = 0.59µ (Fig. 8). The _Photinus_ light extends much further into the red and it is easy to