and Dilatation upon, the Retina. 91 



by the finger, and that its pressure from within outwards will 

 dilate the part of the retina under tlie finger which was for- 

 merly compressed, and will compress all that part of the retina 

 in contact with the elevated ring. An increaseof pressure will 

 be resisted by the opposite part of the retina, and will thus 

 produce a compression at both extremities of the axis of pres- 

 sure, occasioning the diametrically opposite spot of light, and 

 also the luminous spot in the middle of the circular black 

 space. Hence we conclude, that 'when the retina is dilated 

 under exposure to light it becomes absolutely blind, or insensible 

 to all luminous impressions. 



These properties of the retina often exhibit themselves invo- 

 luntarily when the body is in a state of perfect health. When 

 we move the eyeball by the action of its own muscles, the 

 retina is affected beneath the place where the muscles pull 

 the eyeball; and there may be seen opposite each eye and 

 towards the nose two semicircles or crescents of light ; and 

 other two extremely faint towards the temples. At particu- 

 lar times when the retina is more sensible than at others, these 

 crescents become complete circles or rings of light. From the 

 same cause, in the act of sneezing, gleams of light are emitted 

 from each eye, during both the inhalation of the air and its 

 subsequent expulsion ; and in blowing air violently through 

 the nostrils two patches of light appear above the axis of the 

 eye, and in front of it; while other two luminous spots unite 

 into one, and appear about the point of the nose, when the 

 eyes are turned in that direction. 



The phaenomena which have been described are those pro- 

 duced by the parts of the retina which are most affected by 

 any given pressure ; but it is obvious that this pressure is pro- 

 pagated over the whole retina; — and it is a curious fact, that 

 though this pressure is too weak to produce a luminous im- 

 pression, it has yet power to modify other impressions pre- 

 viously made upon the membrane. If, from looking at the 

 sun, the eye .sees a pinkish-broitm spectrum, a pi-essure upon 

 another part of the retina will change it to a green spectrum ; 

 and when the pressure is removed it will again become brown. 

 If the pressure is such as to diminish the sensibility of the 

 retina, it will either diminish or entirely remove a weak spec- 

 tral impression. 



When the eye is pressed in front by putting the finger oil 

 the eyelid above the cornea, no luminous .spectrum is seen; 

 and 1 have not ventured to nuike the i)re.ssure sulliciently 

 strong to make an impression on the back of the eye. I know a 

 case, however, in which this efiect was produced accidentally. 

 A person, in a state of intense grief, had been sitting for some 

 time with his huud pressed against his eye;— the niomenl the 



