Behera im Crystalline Lens in Man. 127 
for him not to become aware of the phenomenon. Thus when I 
look at the nearer of two trees placed several yards apart nearly in 
the same line, in a distant field, the minor distinctness of the farther 
tree is so slight that I may fail to notice it ; but when I look at a 
book through a veil, both being near me and only a few inches 
apart, I find that when my eye is fixed on the print I see it quite 
distinctly, while I am scarcely conscious of the presence of the 
intervening veil; and again, when I look intently at the veil and 
perceive its texture distinctly, at that same moment the print becomes 
confused and unrecognizable. What is the explanation of this ? 
In order to see an object distinctly, an exact image of it must 
be formed on the bacillary layer of the spectator’s retina. Every 
luminous point in the surface of the object turned towards the 
spectator must be represented by a corresponding image-point upon 
his retina. This is effected by the refractive power of the eye—that 
power its transparent parts possess in common with inanimate trans- 
parent bodies, of changing the original direction of the luminous 
rays which enter it, and of giving these rays a new direction of a 
kind dependent on their relative densities and their curves. 
Luminous pencils coming from remote objects consist of parallel 
rays, and, having regard to the small opening of the pupil, the 
pencils which enter the eye from an object 20 or more feet distant 
may be considered to be composed of parallel rays. Now the re- 
fractive power of the eye is such that parallel rays entering it are 
collected in exact foci upon its retina without the exercise of any 
vital effort, the eye itself being quite passive. It would occur as 
well in a dead eye so long as its media remained transparent and 
while they retained their proper curves. 
The luminous pencils which a near object sends to a spectator’s 
eye consist of divergent rays, and the unaided refractive power of 
the eye which sufficed to unite the parallel rays from a distant 
object in the bacillary plane of the retina, is insufficient to collect 
divergent rays in exact foci in this plane. The foci of these rays 
lie behind the retina which the pencils strike as spots, the sections 
of cones, called circles of dispersion, and not as points. The result 
of this is a blurred confused image, and not a clear one, the pro- 
duction of which requires the rays to be brought to exact foci in 
the bacillary layer. 
We are, however, conscious that we possess the power of seeing 
distinctly near objects as well as distant ones, which proves that the 
eye has the power to unite divergent as well as parallel rays in 
exact retinal foci; and this implies the possession of a power of 
altering its refractive state so as to suit it to the distance of the 
object we desire to see distinctly, or, in other words, to adapt it to 
the degree of divergence of the luminous rays entering the eye from 
the object. 
