EFFECTS OF PRESSURE ON THE RETINA. 105 



bright flash of light shoots from the eyeball. In the ac* 

 of sneezing, gleams of light are emitted from each eye, 

 both during the inhalation of the air, and during its 

 subsequent protrusion, 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 as it were 

 about the point of the nose when the eyes are directed to 

 it. When we turn the eyeball by the action of its own 

 muscles, the retina is affected at the place where the 

 muscles are inserted, and there may be seen opposite 

 each eye and towards the nose, two semicircles of light, 

 and other two extremely faint towards the temples. At 

 particular times, when the retina is more phosphorescent 

 than at others, these semicircles are expanded into 

 complete circles of light. 



In a state of indisposition, the phosphorescence of the 

 retina appears in new and more alarming forms. When 

 the stomach is under a temporary derangement accom- 

 panied with headache, the pressure of the blood-vessels 

 upon the retina shows itself, in total darkness, by a faint 

 blue light floating before the eye, varying iii its shape, 

 and passing away at one side. This blue light increases 

 in intensity, becomes green and then yellow, and sometimes 

 rises to red, all these colours being frequently seen at 

 once, or the mass of light shades off into darkness. When 

 we consider the variety of distinct forms which in a state 

 of perfect health the imagination can conjure up when 

 looking into a burning fire, or upon an irregularly shaded 

 surface,* it is easy to conceive how the masses of coloured 



* A very curious example of the influence of the imagination in 

 creating distinct forms out of an irregularly shaded surface, is 

 mentioned in the life of Peter Heaman, a Swede, who was executed 

 for piracy and murder at Leith in 1822. We give it in his own 

 words : [" Ona 



