502 



Dr. G. J. Burch. On Light- 



[Apr. 19, 



On first glancing at the cloud gap the rate dx/dt at which exciting sub- 

 stances were formed was greater than that dn/dt at which sensitive material 

 could be secreted. Accordingly, after a short time of almost painful 

 brilliancy, the store n was reduced to zero, and a hand-to-mouth condition 

 of things set in, during which dx/dt = dn/dt, i.e. just as much exciting 

 substance was formed as could be furnished by the sensitive material 

 secreted. But, during the interval, dx/dt was so far in excess of the normal 

 that maximum sensation was produced both by the light of the clouds and 

 by the sun itself. And it can hardly be denied that during the so-called 

 retinal fatigue, after the details became visible, a condition of very great 

 activity existed.* On the other hand, the sensation was undoubtedly less, 

 and became less as the eyes got accustomed to it. 



Similar phenomena may be seen in a furnace — a blinding glare in which 

 details of flame and of molten metal with slag floating on it gradually appear 

 — or with far less intensity of illumination in the '' faces in the fire " in the 

 hot coals of an open grate. 



There is at first just the same sense of being dazzled — the same gradual 

 perception of details, and in the end the same quiet contemplation of what 

 has become comfortably visible, whether the experiment is made with the 

 hot coals of the open grate or the far greater intensity of the evening sun. 

 Yet the actual intrinsic luminosity of the hot coals is a good deal less 

 than that of the newspaper which we read out of doors in the sunshine on a 

 summer's day. 



The name retinal fatigue for this state is not very apt. It is a condition 

 in which a powerful stimulus produces a reduced effect, not because the 

 organ is in any way deteriorated or used up, but because it works best in 

 that way. It suggests strongly the use of a shunt with a galvanometer. 

 And if we add the idea that with the eye it takes some little time to put 

 the shunt on, and still longer to take it off again, we have a fairly accurate 

 description of the facts. 



The condition in which a shunt factor too strong for the exciting light 

 persists is called the negative after-effect. Thus, if S t = the shunt-factor at 

 time t, 



ch/dt _ j.j ie g^j.gjj-^ f sensa ti on a t that moment. 



This view of the negative after-effect agrees with the theory of Fechner 

 on the subject, as described by Helmholtz.f I have been unable to consult 



* Of. Waller, 'Phil. Trans.,' 1897, B, vol. 188, p. 65, note, "The retina resembles nerve 

 with respect to its inexhaustibility." 



t Helmholtz, ' Handbuch der Physiologischen Optik,' 2nd ed., p. 534. 



