On Light, 389 



and bnlliant^ circlet of light ; some^uhat irregular, and a 

 little yellower than that of the sun. This fact I attribute 

 to the increased pressure on this part of both eyes. 



All persons, who look at the sun, will perceive a bright 

 image of this luminary, remaining in their eyes after they 

 are turned away ; more vivid, when the eyelids are shut ; 

 less, and less, when they are open. Persons, who have 

 weak eyes, will derive this image from objects, which 

 arc in a moderate degree luminous ; and will perceive it 

 much longer, than those, whose eyes are not impaired. 

 On my own such an image is impressed by every thing, 

 which has any lustre ; and, when they are most diseased, 

 by every thing, which is of a light colour. When the 

 image is vivid ; it continues several minutes ; but be- 

 comes gradually fainter, until it disappears. If, during 

 the progress of its decay, I close my eyelids so, as to 

 press them somewhat hardl}^ ; the image always becomes 

 somewhat brighter, than it was immediately before. 

 When this pressure is removed ; it immediately becomes 

 fainter. 



From the uneasiness, which this disease very often oc- 

 casions, I have not unfrequently been induced to press 

 my eyes with my fingers. When the pressure reaches 

 to a certain degree ; the whole field of vision becomes in- 

 stantaneously luminous and bright ; resembling i7i appear- 

 ance a circular, polished plate of silver ; covered with 

 small drops of water, glittering with its lustre. If these 

 drops should be supposed suddenly to rise from behind 

 the plate, and to move on its surface with rapidity, and 

 without much regularity, the resemblance will, I think, 

 be nearly complete. Sometimes I have increased the 

 pressure beyond this degree. In this case, the drops of 

 light have immediately disappeared; and the field of vis- 

 ion has assumed exactly the aspect of the sun ; circular in 

 its figure ; uniform in its appearance ; equally bright ; 

 and the light exacdy of the same hue. 



In some of these instances I have brought my finger 

 suddenly, and carelessly, against the ball of the eye. In 

 consequence of the pressure, produced in these cases, a 

 fiood of light has instantaneously overspread tbe field of 

 vision ; exhibiting all the colours of the rainbow, not 

 W 



