OCULAR SPECTRA. 



moderate its brilliancy, and keep the eye upon it 

 steadily for a few seconds, we shall see, even for 

 hours afterwards, and whether the eyes are open 

 or shut, a spectrum of the sun varying in its 

 colours. At first, with the eye open, it is brown- 

 ish-red with a sky-blue border, and when the eye 

 is shut, it is green with a red border. The red 

 becomes more brilliant, and the blue more vivid, 

 till the impression is gradually worn off; but even 

 when they become very faint, they may be revived 

 by a gentle pressure on the eyeball. 



Some eyes are more susceptible than others of 

 these spectral impressions, and Mr. Boyle men- 

 tions an individual who continued for years to see 

 the spectre of the sun when he looked upon bright 

 objects. This fact appeared to Locke so interest- 

 ing and inexplicable, that he consulted Sir Isaac 

 Newton respecting its cause, and drew from him 

 the following interesting account of a similar 

 effect upon himself: "The observation you 

 mention in Mr. Boyle's book of colours, I once 

 made upon myself with the hazard of my eyes. 

 The manner was this : I looked a very little while 

 upon the sun in the looking-glass with my right 

 eye, and then turned my eyes into a dark corner 

 of my chamber, and winked, to observe the im- 

 pression made, and the circles of colours which 

 encompassed it, and how they decayed by degrees, 

 and at last vanished. This I repeated a second 

 and a third time. At the third time, when the 

 phantasm of light and colours about it were 

 almost vanished, intending my fancy upon therii 

 to see their last appearance, I found, to my 

 amazement, that they began to return, and by 

 little and little to become as lively and vivid as 

 when I had newly looked upon the sun. But 



