OBLIQUE VISION FEEBLE LIGHT. 15 



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 com- 

 pensated 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 dis- 

 appears. 



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 ob- 

 served in daylight by the sportsman when he 

 endeavours to mark upon the monotonous heath 

 the particular spot where moor-game has alighted. 



