| Line | Color | Wave-lengths (µµ = µ/1000) | Source |
|---|---|---|---|
| A | Red | 759.4 (band) | Oxygen in atmosphere. |
| a | Red | 718.5 (band) | Water vapor atmosphere. |
| B | Red | 686.7 | Oxygen vapor atmosphere. |
| C | Red | 656.3 | Hydrogen in sun. |
| D1 D2 | Yellow | 589.6, 589.0 | Sodium in sun. |
| E | Green | 527.0 | Calcium in sun. |
| b1 b2 b4 | Green | 518.4, 517.3, 516.8 | Magnesium in sun. |
| F | Blue | 486.1 | Hydrogen in sun. |
| G | Violet | 430.8 | Calcium in sun. |
| H K | Violet | 396.9, 393.4 | Calcium in sun. |
BUNSEN FLAME LINES
| Source | Color | Wave-lengths (µµ = µ/1000) |
|---|---|---|
| Potassium | Red | 769.9, 766.5 (double) |
| Lithium | Red | 670.8 |
| Sodium | Yellow | 589.6, 589.0 (double) |
| Thallium | Green | 535.1 |
| Magnesium | Green | 518.4 |
| Strontium | Blue | 460.7 |
PLÜCKER TUBE LINES
| Source | Color | Wave-lengths (µµ = µ/1000) |
|---|---|---|
| Mercury | Yellow | 579.0, 576.9 |
| Green | 546.1 | |
| Blue | 491.6, 435.8 | |
| Violet | 407.8, 404.7 | |
| Hydrogen | Red | 656.3 |
| Blue | 486.1, 434.1 | |
| Helium | Red | 728.2, 706.5, 667.8 |
| Yellow | 587.6 | |
| Green | 504.8, 501.6, 492.2 | |
| Blue | 471.3, 447.2 | |
| Violet | 438.8, 402.6, 388.8 |
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 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 spec
tral 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.