316 Sir W. de W. Abney. On the Variation in 



Table XII. 



From Table IX. 



From Table X. 



From Table XI. 



Scale 



Gradation 



Scale 



Gradation 



Scale 



G-radation 



number. 



factor. 



number. 



factor. 



number. 



factor. 



55 '4 



1 38 



39 -3 



1 -10 



47 '4 



1 -20 



40-6 



1-11 



25 



1-00 



32 -9 



1 -07 



31 -4 



1 -06 



15 



1 -02 



22-8 



1 -oo 



22 2 



1-00 



6-6 



1-10 



14 -5 



1-04 



The " G-radation factor" is the alteration required in the abscissa when expressed 

 in powers of 2, the scale No. 22*2 having abscissa of unit length. 



Table XIIL— Exposures 

 to Amyl-acetate. 



No. 



- 



Time in 

 seconds. 



1 



1 



2 



2 



3 



4 



4 



8 



5 



16 



6 



32 



7 



64 



8 



128 



9 



256 



Table XI Y. — Exposures for 

 Monochromatic Rays. 





Time in 



No. 



minutes and 





seconds. 



1 



5" 



2 



10" 



3 



20" 



4 



40" 



5 



1' 20" 



6 



2' 40" 



7 



5' 20" 



8 



10' 40" 



Experiments with Fixed Intensities of Rays, and Times of Exposure varied 

 by means of a Rotating Disc. 



Still one more plan, however, remained to be tried, viz., with a 

 fixed intensity of light, but an alteration in the time of exposure by 

 rotating a disc with gradually increasing apertures before the plate. 

 The disc so pierced is shown in fig. 7. It will be seen that there 

 are two apertures, one near the centre and another at the extreme 

 outside of the radius, which include 40° only. There are thus three 

 apertures of 40°, and if the patch of light is uniform the readings of 

 the three should be the same. All the plate was covered by a mask 

 except a portion J-inch wide which extended its whole length, so that 

 successive portions might be exposed to rays of different wave-lengths 

 at first. The exposed strip of plate was placed in a horizontal 

 direction, i.e., a direction at right angles to the edges of the prisms, 

 and it was then found that the three readings of the 40° apertures 



