€34 The Fundamental Law for th$ true 



than Porter and Slade's method of finding the gradation 

 curves of "" conjugate " plates. Here I will restrict myself 

 to a criticism of some of their statements and to pointing out 

 one or two passages which in my opinion seriously mis- 

 represent the views and the practice of by far the majority 

 of present-day workers. 



Three obvious slips of the pen first deserve mention. 

 On page 187 the last part of the quotation from Hurter & 

 Driffield's paper should read : (" More correctly, the density 

 is a linear function of the logarithm of the intensity of light 

 and time of exposure "). On p. 188, line 7 should read : 

 "through the negative is PT 1 = E 2 :' J ' etc. Finally, on 



p. 189 for T> 2 = ~ read D 2 = lognr, 



In their opening statement of the problem to be solved 

 they employ the words "exposure," " printing light " and 

 w ' viewing light/' and the symbols E l5 E 2 , P and V for 

 thena, when they mean the light intensities acting at each 

 stage of the process. A little later they neglect to state 

 that the abscissae of characteristic curves, whether of 

 negative or positive materials, are not logarithms of light 

 intensities but logarithms of the products of intensity and 

 time, and that the times of exposure for the negative 

 and positive will usually be very different. They also 

 fail to note that the intensity of the light which produced 

 the negative is not equal to that emitted by the original 

 but some fraction thereof depending on the lens and stop 

 etc., used. 



These omissions appear to have somewhat clouded their 

 own vision, as well as made their paper difficult to follow, 

 for they subsequently deduce some strange conclusions 

 concerning the interpretation of their conjugate diagrams. 

 For instance on p. 190, line 4 : '* The above equation will 

 enable us to. determine the values of Jj 2 and logE 2 for 

 a plate that will form a suitable positive, " no indication 

 being given that the value of logE 2 must depend on the 

 sensitiveness of the positive material used. 



Again, on p. 191 we read: "It must not be forgotten 

 that the constant amounts by which it [i.e. the origin] 

 is shifted in the two directions are connected with the- 

 viewing light and printing light. In fact, 



00'= log [viewing light -r-K] =log V — log K, 

 0'0"= log [printing light] = log P." 



