236 POPULAR SCIENCE MONTHLY. 



when their colors are different. An additional source of uncertainty 

 is brought in by what is known as the Purkinje phenomenon, after the 

 physicist who first observed it. He found that if we took two lights of 

 equal apparent brightness, the one red and the other green, and then 

 increased or diminished them in the same proportion, they would no 

 longer appear equal. In other words, the geometrical axiom that halves 

 or quarters of equal quantities are themselves equal, does not apply to 

 the effect of light on the eye. If we diminish the two equal lights, we 

 find that the green will look brighter than the red. If we increase 

 them in the same proportion, the red will look brighter than the green. 

 In other words, the red light will, to our vision, increase or fade away 

 more rapidly with a given amount of change than the green light will. 



It is found in recent times that this law of change does not extend 

 progressively through all spectral colors. It is true that as we pass 

 from the red to the violet end of the spectrum the yellow fades away 

 less rapidly with a given diminution than does the red, and the green 

 still less rapidly than the yellow. But when we pass from the green 

 to the blue, it is said that the latter does not fade out quite so fast 

 as the green. 



One obvious conclusion from all this is that two stars of different 

 colors which look equal to the naked eye will not look equal in the 

 telescope. The red or yellow star will look relatively brighter in a 

 telescope; the green or bluish one relatively brighter to the naked eye. 



In recent times stars have been photographed on a large scale. 

 Their magnitudes can then be determined by the effect of the light 

 on the photographic plate, the impression of the star, as seen in a 

 microscope, being larger and more intense as the star is brighter. But 

 the magnitude thus determined is not proportional to the apparent 

 brightness as seen by the eye, because the photographic effect of blue 

 light is much greater than that of red light having the same apparent 

 brightness. In fact, the difference is so great that, with the chemicals 

 formerly used, red light was almost without photographic effect. Even 

 now, what we measure in taking the photograph of a star is almost 

 entirely the light in the more refrangible portions of the spectrum. It 

 appears, therefore, that when a blue and a yellow star, equally bright 

 to the naked eye, are photographed, the impression made on the nega- 

 tive by the blue star will be greater than that made by the yellow one. 

 A distinction is therefore recognized between photographic and visual 

 magnitudes. 



The photographic magnitudes of the stars are now being inves- 

 tigated and catalogued on a scale even larger than that on which we 

 have studied the visual magnitudes. Yet we have to admit the non- 

 correspondence of the two systems. The bluer the star, the brighter 

 will be its photographic as compared with its visual magnitude. The 



