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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



it must be increased by a certain constant fraction of its total amount in order 

 to produce a perceptible difference in sensation. This is only a special case of 

 a general law of sensation known as Weber's law, which has been formulated 

 by Foster as follows : " The smallest change in the magnitude of a stimulus 

 which we can appreciate through a change in our sensation always bears the 

 same proportion to the whole magnitude of the stimulus." 



Luminosity of Different Colors. When two sources of light having the 

 same color are compared, it is possible to estimate their relative luminosity 

 with considerable accuracy, a difference of about 1 per cent, of the total 

 luminosity being appreciated by the eye. When the sources of light have 

 different colors, much less accuracy is attainable, but there is still a great differ- 

 ence in the intensity with which rays of light of different wave-lengths affect 

 the retina. We do not hesitate to say, for instance, that the maximum 

 intensity of the solar spectrum is found in the yellow portion, but it is import- 

 ant to observe that the position of this maximum varies with the illumina- 

 tion. In a very brilliant spectrum the maximum shifts toward the orange, 

 and in a feeble spectrum (such as may be obtained by narrowing the slit of 

 the spectroscope) it moves toward the green. The curves in Figure 240 illu*- 



8.8 



3.6 



3.4 



3.2 



3. 



2.8- 



2.6 



2.4 



2.2 



2. 



1.8 



1.6 



1.4 



1.2 



1. 



0.8 



0.6 



0.4 



0.2 



Intensity 



490 



470 



450 



430 

 G 



670 650 625 605 590 575 555 535 520 505 

 BCD E F 



FIG. 240. Diagram showing the distribution of the intensity of the spectrum as dependent upon the 



degree of illumination (Konig). 



trate this shifting of the maximum of luminosity of the spectrum with vary- 

 ing intensities of illumination. The abscissas represent wave-lengths in 

 millionths of a millimeter, and the ordinates the luminosity of the different 

 colors as expressed by the reciprocal values of the width of the slit necessary 

 to give to the color under observation a luminosity equal to that of an arbi- 



