242 THE POPULAR SCIENCE MONTHLY 



retina are selective absorbers of light. Any illumination should, there- 

 fore, 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. Xaturally 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. 



Definitiox of Emission and Absoeption Centees 

 The problem of finding the constitution of the emission and absorp- 

 tion 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 constitu- 

 tion of such centers with the various molecular, atomic, ionic and elec- 

 tronic 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 impos- 

 sible 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. "Wlien 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 

 Eowland 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, t. 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 inde- 

 pendent of each other. Apparently there are a large number of these 

 vibrating S3^stems 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 for- 

 ward to prove that the center is even of atomic magnitude. 



The Peoblem of Light axd Heat Cextees 

 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 tlicy exist are very different from the con- 

 ditions under which we study the other physical and chemical units of 

 matter. Then again, it seems that light centers have a comparatively 



