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Mr. H. Hartridge. Factors Affecting the [Nov. 1, 



also evidence is afforded of the symmetry of the bands. Limiting the change 

 of intensity to that part of the spectrum occupied by one side of a band still 

 causes the same change as those described above, an increase in intensity 

 making that side of the band narrower, a decrease making it broader, the 

 change being always such as will cause the band, as a whole, to appear shifted 

 away from the side of greater intensity (see Table V). 



Table V. — Effect of Changing Intensity of Incident Light on One Side of 



the Bands. 



Normal band. 



Increased intensity. 



Decreased intensity. 



Green. 



Yellow. 



G-reen. 



Yellow. 



5765 



5767 



5760 



5759 



5773 





5768 



5762 



5760 



5771 









5760 



5774 



Gelatin films, stained with methyl orange and malachite green, were used 

 to reduce the intensity of the incident light in the green and orange regions 

 of the spectrum respectively. The increased intensity was obtained by 

 supplementing the light used above by a Nernst lamp, which could be fitted 

 with light filters that transmitted either the orange or green parts of the 

 spectrum. 



(3) It is well known that by decreasing the intensity of the light incident 

 on the retina, a point is ultimately reached where no sensation of light can be 

 obtained. This point is called the " visual threshold." The amount by which 

 the light has to be decreased to reach the threshold depends on the activity 

 of retinal response. This not only varies with the wave-length of the light 

 but also with the stimuli received by the retina previous to the threshold 

 measurement. Thus, when the previous stimulus has been intense, a light of 

 considerably greater intensity than the threshold may cause no sensation. 



Moreover, a stimulus of a certain wave-length not only affects the threshold 

 for that wave-length, but also to a less extent that of wave-lengths on either 

 side ; in cases where the previous stimuli have been very intense and 

 prolonged every part of the spectrum becomes affected. 



The position of the visual threshold at different points of the spectrum is 

 one of the factors which determine the appearance of absorption bands ; this 

 is well shown in the case of oxyhemoglobin in Table VI. 



Previous stimulation by a bright light causes the threshold to rise and 

 makes the bands become wider ; a fall in the threshold conversely makes 



