In order to properly understand these lines it is necessary to remember five important things. These are : Firstly, an incandescent solid or liquid body gives out a continuous spectrum ; secondly, an incandescent gaseous body gives out a discontinuous spectrum consisting of bright lines ; thirdly, each element, when in the state of an incandescent gas, gives out lines peculiar to itself ; fourthly, if the light of an incandescent solid or liquid passes through a gaseous body, certain of its rays are absorbed, and black lines in the spectrum indicate the nature of the sub stance which absorbed the ray ; fifthly, each element, when gaseous and incandescent, emits bright rays identical in color and position on the spectrum with those which it absorbs from light transmitted through it. The spectrum of sodium, for instance, shows two bright lines which cor respond in position with the double black line at D (the sodium line). Now, applying these prin ciples to the solar spectrum, we find, from the nature and position of the rays absorbed that its light passes through hydrogen, potassium, sodium, calcium, barium, magnesium, zinc, iron, chromium, cobalt, nickel, copper, and manganese, all in a state of gas, and constituting part of the solar envelope, whence we conclude that these bodies are present in the substance of the sun itself, from which they have been volatilized by heat. The moon and planets have spectra like that of the sun, because they shine by its reflected light, while, on the other hand, each fixed star has a spectrum peculiar to itself. It has,been already stated that the incandescent vapor of each elementary substance has a characteristic spectrum, consisting of fixed lines, which never changes. Spectrum analysis, as this is called, furnishes the chemist with an exquisitely delicate test to enable him to detect the presence of minute quantities of elementary bodies. For instance, by
heating any substance until it becomes gaseous and incandescent, he is able by means of the lines to read off from the spectrum the various elements present in the vapor. By this means several new elements have been discovered.* Photography has been of very great assistance to the spectroscopist by the means it has given him of not only making rapid and correct representation of the spectrum lines, but also in showing him a number of lines of very great importance situated in the ultra-violet parts of the spectrum, and which are totally invisible to the eye. In return for this assistance, spectroscopy has been of great value to photography. In the very earlier days of photography the action of the spectrum upon substances sensitive to light was studied. Indeed, before that period, and as far back as x777, Scheele showed that the violet rays of the spectrum had the greatest darkening action upon silver chloride.
The effect of the spectrum upon photographic sensitive surface is very different from what is apparent to the eye. For instance, yellow and green appear the clearest to us, but the photo graphic plate is hardly affected by them ; but it receives a powerful impression from the indigo and violet rays, which appear dark to our eye, and even from rays that are quite invisible to us. Hence it is not difficult to understand that many-colored objects are represented by photography in a false light. This has been partly remedied by the introduction of the orthochromatic process. By means of so-called ray sensitizers the plate can be made more sensitive to the yellow and red rays and give true representations of colored objects. (See Orthochromatic Photography.
A careful study of the action of the spectrum colors enables us to discover the safest light with which we can illuminate the dark-room for convenience in working. (See Non-actinic Media.)