1912.] Measurement of Ah sorption Bands. 135 



Table VI.— Effect on Band Widths of Altering Value of Threshold. 





"4 per cent. Hb. 



"6 per cent. Hb. 





5829-5712 

 5811-5726 

 5845-5694 



5826-5700 

 5852-5691 

 5859-5680 



After stimulation by a bright light 



The measurements are of the o-band. 



them narrower. Further, if the bands be measured after alteration in the 

 threshold on one side of the band only, then a movement of the mean wave- 

 length of the band will be found to have occurred ; this being in every case 

 towards the side on which the threshold is highest. 



Table VII. — Showing Change in Wave-length of Bands produced by 

 Alterations in Pi,etinal Adaptation. 



CO. 



Normal position of 

 a-band. 



A. 



B. 



per cent. 









24 



5757 -5 



-1-5 



+ 3 



39 -4 



5750 



-11 



+ 2 



10 1 



5762 



-1 5 



+ 21 



53 8 



5739 



-1 



+ 25 



68 -2 



5728 -5 



-1 



+ 36 



The readings in Column A were taken after the eye had been blinded for green rays by a 

 powerful light ; in Column B, the light used was orange. 



Lastly, there is another important factor, viz., contrast, the full effects of 

 which are to receive future consideration. By comparing in several cases 

 the visual appearance of absorption bands with their spectrophotometer 

 curves, the following observations have been made in nearly every case : — 



(1) A certain density gradient is necessary for an absorption band to be 

 recognised. 



(2) Any gradient that exceeds this tends to be increased in slope by 

 contrast. 



(3) Light and dark areas connected by such gradients increase in width. 



(4) Areas of high intensity become brighter, those of low intensity darker. 



(5) Small density changes within such areas tend to become obliterated. 



(6) A line breaking the continuity of a gradient causes a considerable 

 increase in the differences of density on the two sides. 



The method of observation and data will appear at a future date. 



Fig. 3 shows these six effects, it also demonstrates how contrast impairs 



VOL. LXXXVI. — B. L 



