retina are selective absorbers of light. Any illumination should, therefore, be tuned to this selective absorbing mechanism of the eye. Under these conditions the illumination will be most pleasing and there will be a minimum amount of energy used in the emission of the radiation used for illumination. Naturally this kind of radiation will be a "cold" radiation and not a temperature one. It is represented in nature by the light from glow worms and fireflies and in laboratories, approximately, by various kinds of phosphorescent materials, the source of such radiation being at room temperature.
Definition of Emission and Absorption Centers
The problem of finding the constitution of the emission and absorption centers of selective types of spectra such as those of phosphorescent substances, sparks, arcs, flames, etc., is a very difficult one, and at present many efforts are being made to correlate the possible constitution of such centers with the various molecular, atomic, ionic and electronic theories of matter. Emission and absorption centers of light and heat are the smallest particles or entities from which one can obtain any given characteristic emission or absorption spectrum. A further division or change of the centers will result in making it impossible for the given spectrum to be emitted or absorbed although the resultant particles or entities may possess a characteristic spectrum of their own. From the definition it is to be noticed that the centers need not necessarily be matter, i. e., possess mass. "When the centers move with reference to the observer, their spectral lines and bands will show the Doppler effect.
Light centers seem to be very complex in their nature. Professor Rowland used to compare them to a piano and the work of Professor Wood upon resonance and fluorescent spectra indicate that the analogy is quite an appropriate one. Strike a key, i. e., excite a vapor like that of sodium with monochromatic light and a whole set of harmonics will be set into vibration. In the case of sodium vapor, each series of lines or bands seem to be due to vibrations of systems that may be quite independent of each other. Apparently there are a large number of these vibrating systems in the light centers of the fluorescent spectra of sodium. The center itself may correspond to the atom of sodium, though at the present time no definite evidence has been brought forward to prove that the center is even of atomic magnitude.
The Problem of Light and Heat Centers
In the study of light centers, attention must be directed for a moment to the many and serious difficulties connected with the problem of determining the nature and constitution of these particles or entities. The conditions under which they exist are very different from the conditions under which we study the other physical and chemical units of matter. Then again, it seems that light centers have a comparatively