822 EYE 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 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. This law, then, applies to the phenomena happening in the terminal organ, and not, as generally supposed, exclusively to those occurring in the brain. Such electrical phenomena probably result either from thermal or chemical changes in the retina. Recent researches of Boll and Kiihne have shown that light pro duces chemical changes in the retina. If an animal be killed in the dark, and if its retina be exposed only to yellow rays, the retina has a peculiar purple colour, which is at once destroyed by exposure to ordinary light. The purple matter apparently is decomposed by light. Kiihne has also shown that an image may actually be fixed on the retina by plung ing it 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 plate having the sensitive matter quickly removed and re placed by chemical changes ; and it is probable that the electrical expression of these changes is what has been above described. (a) Phosgenes. Luminous impressions may also be pro duced 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 Retinas 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 certain condition of molecular activity, 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 Mariotte in 1668, there is no sensibility to light. Hence, this part of the retina is called the Uind 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 posi tion will be found when the ^ ^ round spot disappears, that * is when its image falls on the entrance of the optic FlG - 15. Diagram for the study nerve.^ There is also com- t tte Blind Spot. plete insensibility to colours at that spot. The diameter of the optic papilla is about 1 S mm., giving an angle of 6 degrees ; 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 (Beaunis). 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 degrees. It is believed that the fossa in the spot, where there are almost 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 dura tion 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 disc having white and black sectors we see continuous dark bands. Even if we paint on the face of the disc 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. It has been ascertained that an im pression lasts on the retina from -^ to -^ of a second. If we look steadily at a bright light for a few seconds and then quickly close the eyes or gaze into a dark room, a luminous image of the light will be visible for a short time. Such an appearance is called a positive accidental image, or a consecutive image. It may also be observed in this ex periment that the intensity of the retinal excitation is nut uniform. It increases quickly at its commencement, and after it has reached a maximum it slowly declines. Many familiar toys, such as the thaumatrope, or wheel of life, stroboscopic discs, and the phenakistoscope, produce curious effects due to persistence of retinal impressions. (e) Tlie 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 the same size. This is due to irradiation, a phenomenon explained by Helmholtz, by stat ing that the borders of clear surfaces advance in the visual field and encroach on obscure surfaces. Probably, even with the most exact accommodation, diffusion images form round the image of a white surface on a black ground, forming a kind of pen umbra, thus causing it to appear larger than it really is. (/) Intensity of Light required to excite the Retina. Light must have a certain intensity to produce a luminous impres sion. It is impossible to fix the minimum intensity FIG. 16. Illustrating the effect of
irradiation.