PRITCHARD S WEDGE PnOTOMETER. 



:Y2 1 



may 



be 



dered 



the 



of a homogeneous ray as the 



twit of tran 



mission for a unit thickness of the absorbing material. As the writer has elsewhere 



shown 



nly not a constant for 



o 



rays, but for these latter 



(though still reckoned for a unit thickness) it increases as the thickness from which 

 it is determined increases. These variations are shown in Fisr. G. 



CURVE 



Showing transmission of homogeneous rays by the 



PRITCHARD PHOTOMETER WEDGE. 



N. B. — The transmission here given is that for the unit 

 layer adopted, or the relative transmission for points in the 

 wedge 1.5 inches apart. 



Abscissae 



Wave-lengths. 



Ordinates = Percentage of transmission by nnit layer. 



Fig. 



In these prismatic measures the light is nearly homo 



be seen, by considering the ratios in 

 single wave-length, that the ratios are 

 tively considerable errors of observati 

 variations due to fluctuations in 



the horizontal line c 

 constant for each ray 

 i where the heat - is 



and 

 pond 



to any 



feebl 



pt for the rela- 

 e. and for the 



the original solar rad 



y 



b 



rge 



the clearest sky 



; * but as we go from one ray to another the ratios differ, 

 and this difference grows very marked as we approach the red end at M .7. 



* That the irregularities due to variations in the solar beam itself are relatively very much greater than those 

 proceeding from purely instrumental causes, may also be inferred from the probable errors attached to the mean 

 deflections of Tables I. and IT; since, if these probable errors are expressed as percentages of the corresponding 

 deflections, it will be seen that they are but slightly greater for the small deflections than they are for the large ones. 



