DISTINCT VISION. 263 



than the central ones; and the suggestion carries with it an air of veri- 

 similitude. Sir David Brewster, however, from the result developed 

 by his experiments, that, "in the case of indirect vision, a luminous 

 object does not vanish, but is seen indistinctly, and produces an en- 

 larged image on the retina, besides that which is produced by the defect 

 of convergency in the pencils," concludes somewhat mystically, "that 

 a star, seen indirectly, will affect a large portion of the retina from 

 these two causes, and, losing its sharpness, will be more distinct." 1 



In order that the image of any object may impress the retina, and 

 be perceived by the mind, it must, first of all, occupy a space on the 

 retina sufficiently large for its various parts to be appreciated : in the 

 next place, the image must be distinct or sharp, in other words, the 

 luminous rays that form it must converge accurately to a focus on the 

 retina : and lastly, the image must be sufficiently illuminated. Each 

 of these conditions varies with the size of the body, and the distance at 

 which it is from the eye ; and there are cases, where they are all want- 

 ing, and the object is consequently invisible. An object may be so 

 small, that the eye cannot distinguish it, because the image, formed on 

 the retina, is too minute. To remedy this inconvenience, the object 

 must be brought near to the eye, which increases the divergence of the 

 rays and the size of the image ; but if we approach it too close to the 

 eye, the rays are not all brought to a focus on the retina, and the 

 image is indistinct. If, therefore, an object be so small, that, at the 

 visual point, to be presently mentioned, the rays proceeding from it 

 do not form an image of sufficient size on the retina, the object is 

 not seen. To obviate this imperfection of the sense, minute bodies 

 may be viewed through a small hole in a piece of paper or card, or 

 with the instrument called a microscope. By looking through the small 

 aperture in the paper or card, the object may be brought much nearer 

 to the eye ; the rays of greatest divergence are prevented by the small- 

 ness of the hole from impinging upon the retina ; and the rest are 

 converged to a focus upon that membrane, so that a sharp and distinct 

 impression is received. The iris is, in this way, useful in effecting dis- 

 tinct vision, the most divergent rays being, by the contraction of the 

 pupil, prevented from falling upon the crystalline. 



Any object that does not subtend an angle of the sixtieth of a degree 

 is invisible ; but the visual power differs greatly in individuals. Some 

 eyes are much more capable of minute inspection than others ; and 

 greater facility is acquired by practice. Professor Ehrenberg, how- 

 ever, found, that in regard to the extreme limits of vision, there is little 

 difference among persons of ordinarily good sight, whatever may be the 

 focal distance of their eyes. The smallest square magnitude usually 

 visible to the naked eye, either of white particles on a bLack ground, or 

 of black upon a white ground, is about the 4 Jgth of an inch ; but parti- 

 cles that reflect light powerfully, as gold dust, may be discovered with 

 the naked eye in common daylight, when not exceeding the yVj^th of 

 an inch ; and, when the substance viewed is in lines instead of particles, 

 it may be seen, if held towards the light, when only ^(jth of an inch 

 in diameter. 



1 Op. citat., p. 249. 



