PRITCHARD S WEDGE PHOTOMETER. 



309 



Observations. 



We begin by the first kind of observ 



direct b 



by me 



the 



of 



mentioned, going throu 



o 



the 



I 



ose made in the path of the 

 total beam at the five points 



ds^e fi 



thin end to thick 



d then 



the observation from thick end to th 



peatin 



gressive 



chang 



which may be taking pi 



to eliminate the effect of any pro 



sly 



pheric absorpt 



A 



po 



in the atmos- 

 a seric -. Each 



and hence (in this first class of observ 



if, owing to changes in the sky, the absolute deflections 



differ 



Nine such series are here given, obtained on two days of excellent sky. 



TABLE I. 



Time 10 a.m. to 12.20 p.m 



No Wedge. 



669 

 680 



666 

 674 

 613 

 659 



Mean (a) 661 ± 5.1 



1000 



We<lge 

 at 0.3 in. 



296 

 294 

 292 



298 

 288 



282 

 284 



288 ± 2.5 



435.0 



Wedge 

 at 1.8 in 



133 



138 

 148 



136 



135 



137 ± 1.2 



207.3 



0.477 



Wcilge 

 at 3.3 in 



76.7 



78.3 

 82.1 

 82.0 

 78.4 

 76.9 

 79.0 

 76.8 



78.8 ± 0.5 



1 1 9.2 



0.576 



Wedge 

 at 4.8 in. 



53.2 



51.7 

 53.4 

 55.5 



52.2 

 51.6 



50.9 

 50.0 



52.3 ± 0.4 



79.1 



0.664 



at c 3 in. 



38.8 

 38.0 

 88.2 



38.8 

 37.2 

 38.5 



37.1 

 38.6 



38.2 ± 0.2 



57.8 



0.731 



Series 1 



Series 2 



Series 3 



Series 4 



(a) are the actual mean deflections observed on the galvanometer, in units of one millimeter. 



(b) are the corresponding proportional value of the original beam. 



(c) are quotients obtained bywlividing the number in (b), giving absorption at each thickness of the wedge, by 



the preceding, thus : 



207.3 



477 119.2 



57G 



435.0 1000 ' 207.3 1000' 



etc. 



Extract from Original Record. 



Station, Allegheny. 



Date, August 2, 1886. 



Temperature of Apparatus — 24° C. at 10 a.m. 



State of Sky, Good blue with a very few patches of cirrocuraulus. Cumuli forming toward noon 



Aperture of Slit = 2 mm. (Slit 11mm. high.) 



