MEMOIR VI.] HYPOTHESES OF VISION. 1Q7 



firmed, the millionth of a second, can without any diffi- 

 culty be photographed even on so sluggish a compound 

 as silver iodide. On the far more sensitive retina the 

 chemical impression must be practically contemporane- 

 ous with the impinging of the light. 



If after the eyelids have been closed for some time, 

 we suddenly and steadfastly gaze at a bright object, 

 and then quickly close the lids again, a phantom image 

 is perceived existing in the indefinite darkness before 

 us. By degrees the image becomes less and less distinct; 

 in a minute or two it has disappeared. 



The chemical hypothesis renders a very clear explana- 

 tion of this effect an explanation that commends itself 

 to our attention as casting light in many cases on the 

 curious phenomena of apparitions: phenomena that have 

 been not without influence on the history of mankind. 



The duration and gradual extinction of the retinal 

 phantoms correspond to the destruction and renovation 

 taking place in the retina itself. The blood supply is 

 very ample, as are likewise the channels for the removal 

 of waste, but the operations require time to be accom- 

 plished. As in machines contrived by man, so in natural 

 organs, the practical working does not always come up 

 to the theoretical standard. Theoretically, as the retina 

 suffers change under the incident light, the removal of 

 waste and nutrition should go on in an equal manner 

 both as to time and quantity. A marvellous approach 

 to the ideal perfection is attained, for though the action 

 of light must necessarily be cumulative, that is, increas- 

 ing with the continuance of exposure, objects do not 

 become brighter and brighter as we look at them, but 

 they attain their predestined distinctness at once. The 

 action of the light, the removal of the waste it is occa- 

 sioning, and the supply for renovation are all contem- 

 poraneously going on with an equal step, or so nearly 



