262 SENSE OF SIGHT. 



spectral illusions is the brain, and that the retina is no farther con- 

 cerned than it is in dreaming or in the hallucinations of insanity. 



The retina is able to receive visual impressions over its whole sur- 

 face, but not with equal distinctness or accuracy. When we regard 

 an extensive prospect, that part of it alone is seen sharply, which falls 

 upon the central part, or in the direction of the axis of the eye : we 

 always, therefore, in our examination of minute objects, endeavour to 

 cause the rays from them to impress this part of the retina; the dis- 

 tinctness of the impression diminishing directly as the distance from 

 the central foramen increases. This central point, called the point of 

 distinct vision, is readily discriminated on looking at a printed page. 

 It will be found, that although the whole page is represented on the 

 retina, the letter to which the axis of the eye is directed is alone sharply 

 and distinctly seen ; and, accordingly, the axis of the eye is directed in 

 succession to each letter as we read. In making some experiments on 

 indistinctness of vision at a distance from the axis of the eye, Sir David 

 Brewster 1 observed a singular peculiarity of oblique vision, namely, 

 that when we shut one eye and direct the other to any fixed point, 

 such as the head of a pin, and hence see all other objects within 

 the sphere of vision indistinctly, if one of these objects be a strip of 

 white paper, or a pin lying upon a green cloth, after a short time, the 

 strip of paper or the pin will altogether disappear, as if it were entirely 

 removed, the impression of the green cloth upon the surrounding parts 

 of the eye extending itself over the part of the retina, which the image 

 of the pin occupied. In a short time, the vanished image will re-ap- 

 pear, and again vanish. When the object seen obliquely is luminous, 

 as a candle, it never vanishes entirely, unless its light is much weak- 

 ened by being placed at a great distance; but it swells and contracts, 

 and is encircled with a nebulous halo, the luminous impressions ex- 

 tending themselves to adjacent parts of the retina not directly influ- 

 enced by the light itself. 



From these, and other experiments of a similar character, Sir David 

 infers, that oblique or indirect vision is inferior to direct vision, not 

 only in distinctness, but from its inability to preserve a sustained vision 

 of objects. Yet it is a singular fact, that indirect has a superiority 

 over direct vision in the case of minute objects, such as small stars, 

 which cannot, indeed, be seen by the latter. A mode frequently 

 adopted by astronomers for obtaining a view of a star of the last degree 

 of faintness is to direct the eye to another part of the field, and in this 

 way, a faint star, in the neighbourhood of a large one, often becomes 

 'very conspicuous, so as to bear a certain illumination, and yet it en- 

 tirely disappears, as if suddenly blotted out, when the eye is turned 

 full upon it; and, in this way, it can be made to appear and disappear 

 as often as the observer pleases. Sir J. F. W. Herschel, and Sir 

 James South, who describe this method of observation, attempt to 

 account for the phenomenon by supposing, that the lateral portions of 

 the retina, being less fatigued by strong light, and less exhausted by 

 perpetual attention, are probably more sensible to faint impressions 



1 Op. citat, p. 248. 



