A given light-filter absorbs a constant fraction of each of the radiations in the light which passes through it, whatever may be the intensity of the radiation. For example, if a certain orange filter absorbs 7o per cent of radiation 5,7oo A.U. (yellowish-green) and 55 per cent of light of wave-length 5,900 A.U. (yellow), these propor tions will be absorbed whatever may be the composition of the incident light (white light or a light already coloured), whatever may be its intensity, and irrespective of the position of the filter in the beam. Moreover, any object whatever will appear precisely the same to the eye, whether the filter is placed between the source of light and the object, or directly in front of the observer's eyes, so that the object receives light directly from the source.
If two light-filters are superposed, the trans mission of the two together is, for each radiation, the product of the two transmissions separately. This holds in whichever order the two filters may be placed. For example, if a certain orange-yellow filter transmits 50 per cent of the radiation 5,60o A.U. (pure green), and if another blue-green filter transmits ro per cent of this radiation, then the two filters together, whether placed in contact or not, and in what ever order the light passes through them, will transmit o-5o X o•io = o-o5, i.e. 5 per cent of the radiation under consideration.
1. Sources of Artificial Light. The light emitted by different common forms of illumin ants differs enormously, as regards the relative proportions of the different radiations, from natural white light. The proportion of red radiations is always greater in artificial sources, and the proportion of violet considerably less. The composition of the light emitted by a solid source depends essentially on the temperature of the source. If the temperature of a body be gradually increased it emits first of all infra-red radiation, then red ; the other spectral radiations appear in their order as the temperature rises. Thus only very high-temperature sources emit an appreciable proportion of violet, which is, however, always less than that in solar radiation, since by no known means can a temperature be obtained comparable with that of the sun.
A simple illustration of these facts can be seen in the case of an electric lamp when the voltage is varied, either accidentally or by means of a rheostat connected in the circuit. The light becomes more white and more active on the photographic plate as the voltage is raised.
To a degree of precision quite sufficient for practical needs, one can determine the composi tion of common artificial sources by considering three groups of radiations, blue-violet, green, and red, with limits respectively at 4,950 A.U. and at 5,800 A.U. instead of considering each individual radiation present.
A well-known effect of the particular composi tion of the radiation from these light-sources is the changed appearance of certain colours when examined in artificial light ; blues change to a deep grey ; light greens to yellow ; violets and pink colours to red. This drawback can be overcome by passing the light through a bluish glass, which, however, reduces considerably the luminous intensity.
Special mention should be made of illuminants which consist of a tube containing a gas, made luminescent by the passage of an electric dis charge through it. Two types of such lamps, mercury-vapour lamps and Neon tubes, are in everyday use, giving respectively a greenish blue and an orange-red light. Neither of these sources gives a continuous spectrum, as do those enumerated above, but simply fine isolated lines. The mercury lamp gives, in addition to the ultra violet, several lines in the violet, green and yellow, but nothing in the red. The Neon lamp, on the other hand, gives radiations in all the spectrum regions, but with marked predomin ance of the red.
It is plain that the light of the mercury-vapour lamp cannot be compensated by filtering the radiations present in excess, since it does not contain any red. Endeavours have been made to obtain from it white light by using it in con junction with under-run incandescent electric lamps, or neon tubes, or by using reflectors coloured with rhodamine, which, by fluorescence, transform into red light a part of the ultra-violet by which they arc illuminated (§ 4).