106 Prof.R. W. Wood on Optical Properties of 



of the paper. This beam was reduced in intensity by a 50 per 

 cent, absorbing film of gelatine, of neutral tint, supplied to me- 

 through the kindness of Dr. Mees of the Eastman Kodak Co.. 



■Z^ * d^ rc 



so y 



Absorbing 

 screen 



O Sodium 

 bulb 

 **-- Mirror 

 -. White paper 



mPr 



> Lens 



Hilger 

 spectroscope 



w 



In front of the sodium bulb was mounted a thin sliver of' 

 freshly silvered glass with a plane surface, and razor edge,, 

 obtained by hammering the edge of a piece of silvered plate 

 glass. This sliver was turned so as to reflect light from the 

 portion of the paper illuminated by the light passed by 

 the disk into the eye (or subsequently into the Hilger wave- 

 length spectroscope). The sodium film reflected light from 

 the portion of the paper illuminated by the beam passed by 

 the absorbing film in the same direction. If the eye was 

 placed in the position occupied by the spectroscope, the 

 reflected image of the paper, seen in the concave sodium 

 mirror, could be partly covered by the illuminated sliver of 

 silvered glass. 



Assuming the silver to reflect 100 per* cent., it is clear that, . 

 with this arrangement, if the sodium had a reflecting power 

 of 100 per cent., we should have the fields equally illuminated 

 (shown bv a disappearance of the razor edge of the glass 

 sliver) with the sector wide open, i. e. reading 100 per cent. 



