776 OF'THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



ground, and a black spot of precisely the same dimensions upon a white 

 ground, the former will seem to be considerably larger than the latter ; ap- 

 parently because the excitation of the retina by the luminous impression 

 tends to spread itself in each case over the adjacent non-excited space. 

 Hence it is that we are able to distinguish any small magnitudes, such as 

 letters or the lines of a diagram, at a much greater distance when they are 

 marked in white on a black ground, than when inscribed in black upon a 

 white ground. Another curious case of the same kind has been noticed by 

 Sir D. Brewster. 1 "If we shut one eye, and direct the other to any fixed 

 point, such as the head of a pin, we shall see indistinctly all other objects 

 within the sphere of vision. Let one of these objects thus indistinctly seen, 

 be a strip of white paper or a pen lying on a green cloth. Then, after a 

 short time, the strip of paper, or the pen, will disappear altogether, as if it 

 were entirely removed ; the impression of the green cloth upon the surround- 

 ing parts of the retina, extending over that part of it which the image of 

 the pen occupied. In a short time the vanished image will reappear, and 

 again vanish ; when both eyes are open, the very same effect takes place, but 

 not so readily as with one eye. If the object seen indistinctly is a black 

 stripe on a white ground, it will vanish in a similar mauner. When the 

 object seen obliquely is luminous, such as a candle, it will never vanish en- 

 tirely, until its light is much weakened by being placed at a great distance; 

 but it swells and contracts, and is encircled by a nebulous halo." 



634. The power of receiving and transmitting visual impressions is by no 

 means uniformly diffused over the entire Retina. In the whole field of vision 

 which at any time lies before us, we only see with perfect distinctness that 

 part to which the axes of our eyes are directed, and of which the image, 

 therefore, is formed upon the " yellow spot" ( 616). Nevertheless we have 

 a sufficiently distinct perception of the remainder of the field, to enable us 

 to judge of the relations of the objects which are distinctly seen to those 

 which surround them ; and the mobility of our eyes enables us, under the 

 guidance of our visual sense ( 541), to direct the most sensitive spot of the 

 retina to every part of the field in succession, not only without effort, but 

 even almost without the consciousness that we are doing so. Generally 

 speaking, the indistinctness of vision for objects seen out of the axis of the 

 eye, increases with the distance of their images from the central point; or, 

 in other words, the impressibility of the several parts of the retina dimin- 

 ishes, according to their distance from the " yellow spot." For a small space 

 around it, however, the vision is tolerably accurate, and the extent of this 

 circle of clear "indirect vision," as it is termed, varies in different people; 

 Volkmann, for instance, can read an entire word by the light of a single 

 electrical spark, and must therefore have instantaneously a direct and per- 

 fect perception of every part of it, whilst by others only a letter or two can 

 be distinguished. It appears also from the experiments of Aubert and 

 Forster, 2 and of Heyinann, 8 that when the Eye is accommodated for near 

 objects, the limits of clear lateral perception are increased, or at least the 

 perceptibility of that circle is intensified ; so that of two objects which throw 

 images of equal size on the Retina, the smaller and more approximated one 

 is distinctly perceived at a greater distance from the yellow spot than the 

 larger and more distant object. But there is one part of the retinal surface, 

 namely, the seat of entrance of the Optic Nerve, which is remarkable for 

 its imperfect receptivity ; as is shown by the following experiment. Let two 



Treatise on Optics, in Lardner's Cyclopa-dia, p. 296. 

 1 Archiv f. Ophthalmol., Bd. iii, p. 1. 

 8 Acta Acadein. C.L. C.U. Nature Curiosorum, Dresden, 1864. 



