246 SENSE OF SIGHT. 



If pressure be made on the eyeball, behind the cornea, so as to affect 

 the retina, concentric luminous circles are seen, opposite to the part on 

 which the pressure is applied ; and, if the pressure be continued for 

 twenty or thirty seconds, a broad undefined light, which increases in 

 intensity every moment, rises immediately before the eye. If the eye- 

 lids be open, and light be present on the repetition of the last experi- 

 ment, a dense cloud arises, instead of the broad undefined light ; and 

 the eye becomes, in a few seconds, perfectly blind; but in the course 

 of three or four seconds after the finger is removed, the cloud appears 

 to roll away from before the eye. From this, it seems, that sensations 

 of light may be produced by mechanical pressure made on the retina ; 

 in other words, the retina becomes phosphorescent by pressure. The 

 same thing is observed if a sudden blow be given on the eye, or if we 

 place a piece of zinc under the upper lip, and a piece of copper above 

 the eye. A flash of light is seen ; produced, doubtless, by the galvanic 

 fluid impressing directly, or indirectly, the optic nerve. The same 

 thing occurs in the act of sneezing, and in forcing air violently through 

 the nostrils. On repeating the experiment of pressing the eyeball, Sir 

 David Brewster 1 observed, that when a gentle pressure is first applied, 

 so as to compress slightly the fine pulpy substance of the retina, a 

 circular spot of colourless light is produced, though the eye be in 

 total darkness, and has not been exposed to light for many hours ; but 

 if light be now admitted to the eye, the compressed part of the retina 

 is found to be more sensible to the light than any other part ; and, 

 consequently, it appears more luminous. If the pressure be increased, 

 beyond the point mentioned above, the circular spot of light gradually 

 becomes darker, and, at length, black, and is surrounded with a bright 

 ring of light. By augmenting the pressure still more, a luminous spot 

 appears in the middle of the central dark one, and another luminous 

 spot diametrically opposite, and beneath the point of pressure. " Con- 

 sidering the eye," says Sir David, " as an elastic sphere, filled with 

 incompressible fluids, it is obvious, that a ring of fluids will rise round 

 the point depressed by the finger ; and that the eyeball will protrude 

 all round the point of pressure ; and consequently the retina, at the 

 protruded part, will be compressed by the outward pressure of the con- 

 tained fluid, while the retina on each side, that is, under the point of 

 pressure, and beyond the protruded part, will be drawn towards the 

 protruded part or be dilated. Hence the part under the finger, which 

 was originally compressed, is now dilated, the adjacent parts are com- 

 pressed, and the more remote parts, immediately without this, dilated 

 also." "Now," continues Sir David, "we have observed, that when 

 the eye is, under these circumstances, exposed to light, there is a bright 

 luminous circle shading off externally and internally into total darkness. 

 We are led therefore to the important conclusions, that when the retina 

 is compressed in total darkness it gives out light ; that when it is com- 

 pressed, when exposed to light, its sensibility to light is increased ; 

 and that when it is dilated under exposure to light, it becomes abso- 

 lutely blind or insensible to all luminous impressions." 



1 Letters on Natural Magic, Amer. edit., p. 27, New York, 1832. 



