PRITCHARD S WEDGE PnOTOMETER. 



309 



Observations. 



We begin by the first kind of observations, — those made in the path of the 

 direct beam, — by measuring the transmission of the total beam at the five points 

 mentioned, going through the wedge from thin end to thick, and then repeating 

 the observation from thick end to thin, so as to eliminate the efiect of any pro- 

 gressive change which may be taking place simultaneously outside in the atmos- 

 pheric absorption. A set of two measures on each point constitutes a series. Each 

 series is complete in itself, and hence (in this first class of observation) it is immaterial 

 if, owing to changes in the sky, the absolute deflections in two series difier. 



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



TABLE I. 



Time 10 a.m. to 12.20 p.m. 



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



(6) are the corresponiiing proportional value of the original beam. 



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



the preceding, thus 



207.3 

 435.0 



477 llfl.2 

 1000 ' 207.3 ■ 



570 

 1000 



, etc. 



Extract from Original Record. 



Station, Allegheny. 



Date, August 2, 1886. 



TemjKrature of Apparatus = 24° C. nt 10 a.m. 



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



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



