some Carbonates of the Calcite Group. 337 



different voltages. The smooth curve of fig. 20 is the 

 curve for 50,000 volts. The graphs of fig. 12a show the 

 effect on the plate when the voltage across the tube is 

 respectively (1) 40 K.V., (2) 50 K.V., and (3) 60 K.V. 

 The last, and tallest, (3), is calculated using what seem 

 to be the most probable values at hand. The full curves 

 of figs. 12 b, 0, d are the same ones -as those of 1ig. 12a, but 

 now each is calculated on the basis of unit effect at the 

 point of maximum absorption. These are more useful 

 because it is the shape of these curves that is of import- 

 ance ; greater intensity can always be obtained by a longer 

 exposure. All agree in showing the greatest effect upon 

 the plate at the position of maximum absorption for sil- 

 ver. In the case of the lowest voltage (40,000) the intens- 

 ity is seen to rise rapidly to the maximum (fig. 12d) and 

 then drop, a state of affairs particularly desirable since 

 the radiation here is very nearly monochromatic. The 

 intensity then rises again between the first and second re- 

 flections to another maximum nearly as important as the 

 first, after which it falls off till the critical absorption of 

 bromine is reached, when it takes a further fall. At still 

 lower voltages the continued shift of the maximum in the 

 direction of longer wave lengths would shortly dwarf the 

 silver absorption peak into insignificance compared with 

 the second maximum. This is highly undesirable because 

 only the short wave lengths are useful, for the reason that 

 a reflection in the neighborhood of n\ equal to the longer 

 wave length maximum might be either a first order for 

 this region, the second order of a much shorter wave 

 length, or both together. 



At 50 K.V. the intensity distribution curve has its maxi- 

 mum near the critical absorption value for silver, and as 

 a result the relative effect of this region of wave lengths is 

 great. Fig. 12b shows that the second maximum has 

 become insignificant. As the voltage is made still 

 higher (60 K.V.) the further shift of the intensity curve 

 (fig. 12c) towards short wave lengths results in there 

 being a considerable spectrum range, just shorter than the 

 critical value of silver, over which there is a marked effect 

 upon the photographic plate. At the same time the 

 longer wave length maximum has disappeared. Rais- 

 ing the voltage to a higher value would cause this process 

 of increasing the effect of the shorter wave lengths to 

 continue so that the point would soon be reached where 



