pritcuard's wedge photometer. 



317 



Extract from Original Record. 



Conditions the same as on the previous day, August 9, except the following : 



State of Sky 

 Setting on D 

 Battery Cur) 

 Object 



rf Apparatus = 30 C. at 2 p.m. 



315° 47' 15". (Slit 



0.038 Ampere. 



Good sky between clouds. 







■) 



repetition of measurements of wedge transmission at different points in the spectrum 



Time of first five series 10.15 a.m. to 12.30 p.m. 



u 



last 



a 



" 12.30 p.m. to 1.45 p.m. 



In our measures in the direct beam each series was complete in itself, and h 

 we were independent of any changes of the weather from day to day; but 1 



though the 

 in itself, it 



between different points of the wedge may be also coni[ 



not 



\y true that the comp 



betwe 



different wave- 



lengths made on different days, though at the same point of the wedge, is to be made 

 with equal immunity. 



Owing to variations in the initial solar radiation, due to atmospheric change's 



and the alteration of atmospheric absorption, with the changing altitude of the sun, 

 the 



comparable with each other ; nor, rigorously speaking, can one day be exactly com- 

 parable with another when the progressive absorption in the spectrum is in question, 



foregoing series as represented by the vertical columns are not then so strictly 



rather than the 

 continuous serie 



ate of absorption for different parts of the wedge. The foil 

 made when the sun's altitude above the horizon was 54°, wit 



.- 



the 



wedge set at 0.3 in., will however serve to indicate the relative intensities for 



position. 



TABLE VI. 



Wedge at 0.3 in. 



There is of course no reason why similar series should not be taken for every 

 part of the wedge, except the inordinate time demanded in waiting for days of 

 unexceptionable clearness ; for experience seems to show that with all its drawbacks 

 sunlight is better for this purpose than artificial light. 



