RECENT PROGRESS IN PHYSICS. 



381 



;Obliged to contrive a new photometric principle. In fact he solved 

 ■the problem in a very ingenious manner. 



If a disk be divided into sectors equally large ^'"- "^• 



and alternately black and white, as in fig;. 79, 

 ;5ind be put into rapid rotation, the different 

 sectors cannot be distinguished when the disk 

 is illuminated by a constant source of light ; 

 but if it be illuminated by an electrical spark 

 for an instant the sectors of the rotating disk 

 iwill become visible again, and as much more 

 ,so as the electrical spark is brighter. But if 

 |the illumination by the electrical spark be 

 gradually weakened, while that from the con- 

 stant source of light remain the same, a point will be attained where 

 the sectors just cease to be distinguishable, and in this case the power 

 of the illumination by the electrical spark is a determinate fraction 

 of the illumination by the constant source, its magnitude depending 

 lupon the peculiarity of the observer's eye. 



J We will now consider in what manner this limit of the ability to 

 ! distinguish may be ascertained. 



A part of a sector on a white disk, (fig. 

 [80,) being blackened, and the disk turned 

 rapidly about its centre, the black piece will 

 form a ring somewhat darker than the white 

 ground of the disk. The ring will appear as 

 much fainter as the black spot is narrower, 

 , and if the experiment be made with a series 

 of such disks, each successive one having a 

 narrower black end-portion of a sector m n, 

 I we will at last fin«! one in which the dark 

 ring ceases to be distinguishable. 



Xet us suppose this to be the case when 

 I the breadth of the sector is y^-q of ^be entire 



.circumference; it is evident that the brightness of the ring is less 

 than the brightness of the disk by jl--^ ; in this case the eye cannot 

 j distinguish a difi'erence of yio- ^^ illumination. 



Masson made his experiments with disks upon which the breadth of 

 the sectors were ^oj irV, yVj toj ^\, tj-q, rfoj tIo, of the whole cir- 

 cumference, and by means of them he found that for weak eyes a 

 difference of illumination of Jy- to J-^ was the limit of perceptibility. 



For ordinary eyes this limit was y-V to 



for very good eyes -yoo 



to rfo- 



On varying tlie intensity of the illumination Masson found that 

 the sensibility for the same individual did not change if the illumi- 

 nation was sufficient for reading ordinary print. 



The rotating plate being illuminated with colored light, Masson 

 found that the limit of perceptibility of difference of illumination is 

 independent of the color. 



