OBLIQUE VISION FEEBLE LIGHT. 101 



it never wholly disappears, but it spreads itself out into a 

 cloudy mass, the centre of which is blue, encircled with a 

 bright ring of yellow light. 



This inability of the eye to preserve a sustained vision 

 of objects seen obliquely, is curiously compensated by 

 the greater sensibility of those parts of the eye that have 

 this defect. The eye has the power of seeing objects 

 with perfect distinctness, only when it is directed straight 

 upon them ; that is, all objects seen indirectly are seen 

 indistinctly ; but it is a curious circumstance, that when 

 we wish to obtain a sight of a very faint star, such as one 

 of the satellites of Saturn, we can see it most distinctly 

 by looking away from it, and when the eye is turned full 

 upon it, it immediately disappears. 



Effects still more remarkable are produced in the eye 

 when it views objects that are difficult to be seen from the 

 small degree of light with which they happen to be 

 illuminated. The imperfect view which we obtain of 

 such objects forces us to fix the eye more steadily upon 

 them ; but the more exertion we make to ascertain what 

 they are, the greater difficulties do we encounter to 

 accomplish our object. The eye is actually thrown into a 

 state of the most painful agitation, the object will swell 

 and contract, and partly disappear, and it will again 

 become visible when the eye has recovered from the 

 delirium into which it has been thrown. This phenomenon 

 may be most distinctly seen when the objects in a room 

 are illuminated with the feeble gleam of a fire almost 

 extinguished ; but it may be observed in daylight by the 

 sportsman when he endeavours to mark upon the mono- 

 tonous heath the particular spot where moor -game has 

 alighted. Availing himself of the slightest difference of 

 tint in the adjacent heath, he keeps his eye steadily fixed 

 on it as he advances, but whenever the contrast of illumina- 

 tion is feeble, he will invariably lose sight of his mark, 



