RESEARCHES BASED UPON THE THEORY 223 



t 



We have already described the methods of obtaining the luminosity 

 curve of the spectrum. In it the abscissae are wave-lengths of the 

 spectrum, the ordinates luminosities in arbitrary units, the maximum 

 brightness being 100. 



Suppose that when making a set of observations we start with the 

 movable slit at the extreme end of the spectrum and determine the 

 intensity >j of the white which appears of the same brightness as the 

 colour and then move the slit towards the blue through a distance equal 

 to its width and again determine the intensity w% of the white, and so on 

 throughout the spectrum. We should in this way determine piece by 

 piece the brightness of the whole spectrum, and the sum w^ + w 2 + t0 3 + , 

 etc., could be taken to represent the total brightness of the whole 



Fig. 60. R, G, and B, sensation curves. These arc Konig and Dieterioi's curves cor- 

 rected to new determinations of the points of section, a, b, c, d. Abscissae, wave- 

 lengths of the interference spectrum of the arc light ; ordinates, arbitrary scale. 

 (F. Exner.) 



spectrum. If now the slide carrying the slit were removed, so that the 

 light corresponding to the whole spectrum were allowed to fall on the 

 same portion of the screen, thus forming white, and we now determine 

 the intensity W of the comparison white which is equal in brightness to 

 this recombined spectrum, then it has been shown by Abney, Tufts, 

 Ives and others that 



W = w l + w 2 + w 3 + , etc. 



That is, the luminosity of the recombined spectrum is equal to the sum 

 of the luminosities of its parts. 



Now the sum w 1 + w 2 + w 3 + , etc., is proportional to the area 



