the eye as to secure sharpness of definition of the retinal image. This it accomplishes by (1) diminishing the amount of light reflected from near objects, by cutting off the more divergent rays and admitting only those approaching a parallel direction, which, in a normal eye, are focused on the retina; and (2) preventing the error of spherical aberration by cutting off divergent rays which would otherwise impinge near the margins of the lens, and would thus be brought to a focus in front of the retina.
3. Specific Influence of Light on the Retina
The retina is the terminal organ of vision, and all the parts in front of it are optical arrangements for securing that an image will be accurately focused upon it. The natural stimulus of the retina is light. It is often said that it may be excited by mechanical and electrical stimuli; but such an observation really applies to the stimulation of the fibres of the optic nerve. It is well known that such stimuli applied to the optic nerve behind the eye produce always a luminous impression; but there is no proof that the retina, strictly speaking, is similarly affected. Pressure or electrical currents may act on the eyeball, but in doing so they not only affect the retina, consisting of its various layers and of Jacob's membrane, but also the fibres of the optic nerve. It is possible that the retina, by which is meant all the layers except those on its surface formed by the fibres of the optic nerve, is affected only by its specific kind of stimulus, light. This stimulus so affects the terminal apparatus as to set up actions which in turn stimulate the optic fibres. The next question naturally is—What is the specific action of light on the retina? A. F. Holmgren, and also J. Dewar and J. G. M'Kendrick, have shown that when light falls on the retina it excites a variation of the electrical current obtained from the eye by placing it on the cushions of a sensitive galvanometer. One electrode touches the vertex of the cornea and the other the back of the eyeball. The corneal vertex is positive to the back of the eye, or to the transverse section of the optic nerve. Consequently a current passes through the galvanometer from the cornea to the back. Then the impact of light causes an increase in the natural electrical current—during the continuance of light the current diminishes slowly and falls in amount even below what it was before the impact—and the withdrawal of light is followed by a rebound, or second increase, after which the current falls in strength, as if the eye suffered from fatigue.
It was also observed in this research that the amount of electrical variation produced by light of various intensities corresponded pretty closely to the results expressed by G. T. Fechner's law, which regulates the relation between the stimulus and the sensational effect in sensory impressions. This law is, that the sensational effect does not increase proportionally to the stimulus, but as the logarithm of the stimulus. Thus, supposing the stimulus to be 10, 100 or 1000 times increased, the sensational effect will not be 10, 100 or 1000 times, but only 1, 2 and 3 times greater.
Such electrical phenomena probably result either from thermal or chemical changes in the retina. Light produces chemical changes in the retina. If a frog be killed in the dark, and if its retina be exposed only to yellow rays, the retina has peculiar purple colour, which is at once destroyed by exposure to ordinary light. The purple matter apparently is decomposed by light. An image may actually be fixed on the retina by plunging the eye into a solution of alum immediately after death. Thus it would appear that light affects the purple matter of the retina, and the result of this chemical change is to stimulate the optic filaments; if the action be arrested, we may have a picture on the retina, but if it be not arrested, the picture is evanescent; the purple-matter is used up, and new matter of a similar kind is formed to take its place. The retina might, therefore, be compared to a sensitive photographic plate having the sensitive matter quickly removed and replaced; and it is probable that the electrical expression of the chemical changes is what has been above described.
(a) Phosgenes.—Luminous impressions may also be produced by pressure on the eyeball. Such impressions, termed phosgenes, usually appear as a luminous centre surrounded by coloured or dark rings. Sometimes they seem to be small bright scintillations of various forms. Similar appearances may be observed at the moments of opening or of closing a strong electrical current transmitted through the eyeball.
(b) The Retina's Proper Light.—The visual field, even when the eyelids are closed in a dark room, is not absolutely dark. There is a sensation of faint luminosity which may at one moment be brighter than at another. This is often termed the proper light of the retina, and it indicates a molecular change, even in darkness.
(c) The Excitability of the Retina.—The retina is not equally
excitable in all its parts. At the entrance of the optic nerve,
as was shown by E. Mariotte in 1668, there is no sensibility to
light. Hence, this part of the retina is called the blind spot.
If we shut the left eye, fix the right eye on the cross seen in
fig. 15, and move the book towards and away from the eye,
a position will be found when the
round spot disappears, that is
when its image falls on the
entrance of the optic nerve. There
Fig. 15.—Diagram for the
Study of the Blind Spot.
is also complete insensibility to
colours at that spot. The diameter
of the optic papilla is about 1.8 mm., giving an angle of 6°;
this angle determines the apparent size of the blind spot in
the visual field, and it is sufficiently large to cause a human
figure to disappear at a distance of two metres.
The yellow spot in the centre of the retina is the most sensitive to light, and it is chiefly employed in direct vision. Thus, if we fix the eye on a word in the centre of this line, it is distinctly and sharply seen, but the words towards each end of the line are vague. If we wish to see each word distinctly, we “run the eye” along the line—that is, we bring each successive word on the yellow spot. This spot has a horizontal diameter of 2 mm., and a vertical diameter of 8 mm.; and it corresponds in the visual field to an angle of from 2 to 4°. The fossa in the spot, where there are no retinal elements except Jacob's membrane, consisting here entirely of cones (2000 in number), is the area of most acute sensibility. This fossa has a diameter of only 2 mm., which makes the angle ten times smaller. Thus the field of distinct vision is extremely limited, and at the same moment we see only a very small portion of the visual field. Images of external objects are brought successively on this minute sensitive area, and the different sensations seem to be fused together, so that we are conscious of the object as a whole.
Towards the anterior margin of the retina sensitiveness to light becomes diminished; but the diminution is not uniform, and it varies in different persons.
(d) Duration and Persistence of Retinal Impressions.—To excite the retina, a feeble stimulus must act for a certain time; when the retina is excited, the impression lasts after the cessation of the stimulus; but if the stimulus be strong, it may be of very short duration. Thus the duration of an electrical spark is extremely short, but the impression on the retina is so powerful, and remains so long, as to make the spark visible. If we rotate a disk having white and black sectors we see continuous dark bands. Even if we paint on the face of the disk a single large round red spot, and rotate rapidly, a continuous red band may be observed. Here the impressions of red on the same area of retina succeed each other so rapidly that before one disappears another is superadded, the result being a fusion of the successive impressions into one continuous sensation. This phenomenon is called the persistence of retinal impressions. An impression lasts on the retina from 150 to 136 of a second. The cinematography owes its effects to persistence of retinal impressions.
(e) The Phenomena of Irradiation.—If we look at fig. 16, the white square in the black field appears to be larger than the black square in the white field, although both are of precisely
