644 THE POPULAR SCIENCE MONTHLY. 



the eye. The term bright color is sometimes used in a somewhat 

 analogous sense, but the ideas are so totally different that there is 

 little risk of confusion. 



The practical determination of the second constant is possible in 

 a great many cases ; it presents itself always in the shape of a rather 

 troublesome photometric problem, capable of a more or less accurate 

 solution. The relative brightness of the colors of the solar spectrum 

 is one of the most interesting of these problems, as its solution would 

 serve to give some idea of the relative brightness of the colors, 

 which, taken together, constitute white light. Quite recently a set 

 of measurements were made in different regions of the spectrum by 

 Vierordt, who denoted the points measured by the fixed lines, as is 

 usual in such studies. 1 The following table will serve to give an idea 

 of his results : 



Color. Degree of Luminosity. 



Dark red 800 



Red 4,930 



Red, slightly orange 11,000 



Orange red 27,730 



Orange 69,850 



Yellow 78,910 



Green 30,330 



Cyan blue 11,000 



Blue 4,930 



Ultramarine blue 906 



Violet 359 



" 131 



" 58 



9 



These measurements were made on a spectrum obtained by a glass 

 prism, which, as has been mentioned in a previous chapter, contracts 

 the red, orange, and yellow spaces unduly, and hence increases their 

 illumination disproportionately. It is to be hoped that a correspond- 

 ing set of measurements will soon be made on the normal spectrum, 

 furnished by a ruled plate. If we should multiply the luminosity of 

 the colors in either kind of spectrum by their extent or areas, we 

 should obtain measures of the relative amounts of these several tints 

 in white light. 



By the simple method of rotating disks we can very roughly de- 

 termine the second constant in the case of a colored surface, for ex- 

 ample, of paper tinted with vermilion. A circular disk, about six 

 inches in diameter, is cut from the paper, and placed on a rotation 

 'apparatus, as indicated in Fig. 3. On the same axis is fastened a 

 double disk of black-and-white paper, so arranged that the propor- 

 tions of black-and-white can be varied at will. When the whole is set 

 in rapid rotation, the color of the vermilion paper will of course not 



1 C. Vierordt, Poggcndorfs Annalen, Band cxxxvii., S. 200. 



