90 Sir D. Brewster o?; the Effect of Compression 



the motions excited in the bottom of the eye by light, are they 

 not of a vibratinjT nature?" 



The circle of light referred to in this passage always ap- 

 pears opposite to the point of pressure, and its centre has the 

 same visible direction as that of a ray of light incident on the 

 centre of the compressed portion of the retina. The reason 

 why the phaenomenon is best seen by turning the eye away 

 from the finger is, that the retina is thus brought under the 

 point of pressure ; for if the eye remains at rest, or is turned 

 towards the finger, the luminous circle is either imperfectly 

 seen or disappears altogether, because the finger then presses, 

 either wholly or partly, upon a part of the eyeball beneath 

 which the retina does not extend. Sir Isaac Newton is mis- 

 taken in saying, " that the colours vanish in a second when 

 the eye and the finger remain quiet." They undoubtedly con- 

 tinue as long as the pressure is kept up; and in proof of this 

 I may mention a case which I had occasion particularly to 

 study, in which the patient constantly saw the luminous circle, 

 in consequence of an excrescence on the inside of the eyelid, 

 which produced a continued pressure on the eyeball. 



Sir Isaac Newton has not named the colours which he saw 

 in the luminous circle, any further than by saying that they 

 are like those in the feather of a peacock's tail. Although 

 1 have made the experiment a thousand times, under all varie- 

 ties of circumstances, I have never been able to observe any 

 other circles but black and while ones, with the exception of 

 a general red tinge which is seen when the eyelids are closed, 

 and which is produced by the light which passes through 

 them. 



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 is in total 

 darkness, and has not been exposed to light for many hours. 

 If light is now admitted to the eye, the compressed part of the 

 retina is more sensible to the light than any other part, and 

 consequently appears more luminous. Hence it follows, that 

 a slight compression of the retifia iticr eases its sc7isibility to the 

 light which Jails upon it^ and creates a sensation of light when 

 the eye is in absolute darkness. 



If we now increase the pressure, the circular spot of light 

 gradually becomes darker, and at last black, and is surround- 

 ed with a bright luminous 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. Considering the 

 eye as an elastic sphere filled with incompressible fluids, it is 

 obvious that a ring of fluid will rise round the point depressed 



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