402 ON SENSORIAL VISION. 



recurrence of the colours they assume, which are pecu- 

 liarly rich and various the singular effects of gentle pres- 

 sure on the eye, and partial light admitted through the 

 eyelids in modifying them or in renewing them when ex- 

 tinct all these offer a subject of much attraction and 

 interest. A very interesting memoir on them has been, 

 within these few years, communicated to the Royal 

 Society, by Dr Scoresby; but the subject is far from 

 being exhausted, and it is to the habit of attention to 

 such sensorial impressions, fostered by frequently watch- 

 ing the development of these spectra under a variety of 

 circumstances in my own case, that I attribute my having 

 been led to notice that other class of phenomena of 

 which I shall presently speak, and which from their in- 

 conspicuousness, I suppose, escape the notice of most 

 people. 



(3.) The production of Ocular Spectra refers itself, I 

 presume, to what I have described as the purely physical 

 branch of the general subject of vision. Their seat, it 

 can hardly be doubted, is the retina itself,* and their 

 production is in all probability, part and parcel of that 

 photographic process by which light chemically affects 

 the retinal structure, and of the gradual restoration of 

 that structure to its normal state of sensitiveness by the 

 fading out of the picture impressed. Cases are not want- 

 ing in artificial photography where an impression made 



* In speaking of the retina, I would not be understood to express 

 any opinion on the disputed question whether the retina anatomi- 

 cally so called or the choroid coat of the eye be really the seat of 



