ATMOSPHERIC ATTENUATION OF ULTRA-VIOLET LIGHT. 369 



plate is placed emulsion side down over a small diaphragm, through 

 which light passes from a standard source. The light is then brought 

 to a focus on one junction of a Bi-Ag thermo-couple connected to a 

 sensitive galvanometer. The deflections of the galvanometer, when 

 the diaphragm is covered by the spectral image and when it is covered 

 by the clear plate near the image, give a means of eliminating the effect 

 of fog on the plate and of getting a quantity proportional to the ca- 

 pacity. 



If d is the galvanometer deflection when the image is over the 



diaphragm, and d' the deflection for the clear plate, then = — j, — 



(-1/ 



is proportional to the opacity, cb = O. The density of the silver 



deposit, D = log 0, is proportional to the logarithm of the exposure, 



1 



D = K log E. So we have (cS)'^ = E. Within certain limits we may 

 write E = It^ where ^j is a quantity which must be determined by 

 experiment, as it is a factor of the plate and over any considerable 

 variation of / is by no means constant. Over the range of time and of 

 wave-lengths used p was found to be approximately 0.92. 



In order to determine the range over which the densities were pro- 

 portional to log time, the values of 8, from the galvanometer readings 

 for X3404 taken from a number of plates made in the laboratory, were 

 plotted against log t, where t varied from one to thirty minutes. It 

 was found that between 5 = 0.30 and 8 = 0.65 for this wave-length 

 the curve was practically a straight line, the complete curves resem- 

 bling the familiar D, log E curve. Similar curves were made for the 

 spectral images of the other Cd lines. Since 8 = 0.50 falls very near 

 the middle of the straight portion of each of these curves, the corre- 

 sponding values of log t necessary to produce this particular value of 5 

 for the different wave-lengths, give a means of calculating the intensi- 

 ties of the Cd lines. Of course the spectral sensitivity of the plate 

 enters into the problem here. If the time required for X3404: to regis- 

 ter 8 = 0.50 is taken as a standard, then the time required for X2S81 

 to register 8 = 0.50 must contain a factor a which depends upon the 

 change of sensitivity with wave-length. For the same values of 8, 



since (cS)* = It^, we have for two different spectrum lines R = hj I\ = 



"ry ) gives the transmission of Xo through 



f 2 fl/ 



a long column of air compared to the transmission of Xi as a standard. 

 Since only ratios are desired, the factor a expressing the spectral 

 sensitivity of the plate drops out. 



