677 



These forimilae are only available in cases of underexposure. For 

 correct exposure we can get an expression by applying the empirical 

 formula given by Hukter and Dripfiei.d, This formula (10) "repre- 

 sents the necessary relation between the density and the exposure 

 which must be fulfilled if photography is true to nature" (Hurter). 



This formula slightly modified is: 



D — log—=a+bA[i = a^ + !>^logQ. . . . (10) 



in which n, n^, b and h^ are numerical constants, Aij the total 

 quantity of reduced silver, Q the total energy of the light. If we 

 use the expression for A<j from (5) in this formula we get: 



D = lo<i -"- = a + b Ari (1 — 6-/'0 = a, + b, log Q (I — f-"') . (11) 



in which we have also corrected the value for Q by using (8) and 

 putting in it only that part of Q which really has been absorbed. 



If the results of this discussion represent the facts with sufficient 

 accuracy, we may draw the conclusion, that between light- and 

 lion (gen-negatives still another point of difference should exist. 



We may expect that in cases of the same density a light-negative 

 contains considerably less silver than a Röntgen-negative ; in cases 

 of liglit- and Röntgen-negatives containing the same quantity of 

 reduced silver, the transparency of the latter will invaluably be 

 greater. 



In order to test the truth of these conclusions I asked my 

 assistant Dr. Katz to make a careful quantitative analysis of the 

 amount of silver in a set of larger plates forming together an ex- 

 posure-scale. The 10 plates measuring nearly 10 X 15 cm. were 

 cut from one plate 30 X 40 cm. Five of them were used for the 

 lightscale, the other 5 for the Röntgen-exposure scale. The results 

 of all the measurements are given in table II. (See p. 678), 



The vertical columns contain : Under Plate 42 the number of 

 each plate; under Q the relative quantities of light; under ff we 

 find the reading of the polarisation photometer ; under 2 /t*^c<(/ r/^- the 

 measured density, whereas the next column contains the most pro- 

 bable value for the calculated density, supposing a linear relation between 

 loi/ 1 and logchjff. The following vertical columns contain: the measured 

 quantity of silver on the whole surface of each plate, the exact 

 measured surface, the quantity of silver per square centimeter. In 

 the last column the most probable quantity of silver is given, cal- 

 culated on the sup[)Osition of a linear relation between kn/ Q and 



