Di- Brewster on sonic Affections '.if the Rcihta. 201 



either in motion or at rest, we are naturally anxious to ascer- 

 tain what it is, and our curiosity calls forth all our powers of 

 vision. This anxiety, however, serves only to baffle us in all 

 our attempts. Excited only by a feeble illumination, the 

 retina is not capable of affording a permanent vision of the 

 object, and while we are straining our eyes to discover its na- 

 ture, the object will entirely disappear, and will afterwards ap- 

 pear and disappear alternately.* The same phenomenon may be 

 observed in day light by the sportsman, when he endeavours 

 to mark, upon the monotonous 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 upon it as he advances ; but whenever 

 the contrast of illumination is feeble, lie invariably loses 

 sight of his mark, and if the retina is capable of again taking 

 it up, it is only to lose it again. 



Since the preceding paper was read, Mr Herschel and Mr 

 South -f- have described a very curious fact, which has some 

 analogy with the phenomena now described. 



" A rather singular method," they remark, " of obtaining 

 a view, and even a rough measure of the angles of stars, of 

 the last degree qfj'aintness, has often been resorted to, viz. to 

 direct the eye to another part of the field. In this way, a faint 

 star, in the neighbourhood of a large one, will often become 

 very conspicuous, so as to bear a certain illumination, which 

 will yet totally disappear, as if suddenly blotted out, when 

 the eye is turned full upon it, and so on, appearing and dis- 

 appearing alternately, as often as you please. The lateral 

 portions of the retina, less fatigued by strong lights, and less 

 exhausted by perpetual attention, are probably more sensible 



* An analogous phenomenon, but arising from a quite different cause, 

 must have often been observed by persons who arc very long-sighted. In 

 a dark night, the pupil dilates to such a degree as to deprive the eye of 

 its power of adjusting itself to moderate distances. (Soe this Journal, 

 vol. i. p. 80.) Hence, if an object presents itself within that distance, 

 the observer must see it with a degree of indistinctness which cannot fail 

 to surprise him, especially as all distant objects, particularly those seen 

 against the sky, will appear to him with their usual sharpness of outline. 



t Seethe r/til. Trans. lS'2t, part hi, p. Ifi, and page '>*3of tliis Number. 



