Psychology · William James
FOOTNOTES:
The eyeball is thus divided into two parts, the anterior one containing the iris, lens, vitreous body, etc., whilst the posterior one contains most of the retina. The two parts can be separated by immersing the eyeball in water, cornea downwards, and simply pulling off the portion to which the optic nerve is attached. Floating this detached posterior cap in water, the delicate retina will be seen spread out over the choroid (which is partly iridescent in the ox tribe); and by turning the cup inside out, and working under water with a camel's-hair brush, the vessels and nerves of the eyeball may be detected. The anterior part of the eyeball can then be attacked. Seize with forceps on each side the edge of the sclerotic and choroid (not including the retina), raise the eye with the forceps thus applied and shake it gently till the vitreous body, lens, capsule, ligament, etc., drop out by their weight, and separate from the iris, ciliary processes, cornea, and sclerotic, which remains in the forceps. Examine these latter parts, and get a view of the ciliary muscle which appears as a white line, when with camel's-hair brush and scalpel the choroid membrane is detached from the sclerotic as far forward as it will go. Turning to the parts that cling to the vitreous body observe the clear ring around the lens, and radiating outside of it the marks made by the ciliary processes before they were torn away from its suspensory ligament. A fine capillary tube may now be used to insufflate the clear ring, just below the letter _p_ in Fig. 3, and thus to reveal the suspensory ligament itself.
All these parts can be seen in section in a frozen eye or one hardened in alcohol. [11] This vertical partition is introduced into stereoscopes, which otherwise would give us three pictures instead of one. [12] The simplest form of stereoscope is two tin tubes about one and one-half inches calibre, dead black inside and (for normal eyes) ten inches long. Close each end with paper not too opaque, on which an inch-long thick black line is drawn. The tubes can be looked through, one by each eye, and held either parallel or with their farther ends converging. When properly rotated, their images will show every variety of fusion and non-fusion, and stereoscopic effect. [13] Martin: The Human Body, p. 530. [14] Ibid. [15] The ordinary mixing of _pigments_ is not an addition, but rather, as Helmholtz has shown, a subtraction, of lights. To _add_ one color to another we must either by appropriate glasses throw differently colored beams upon the same reflecting surface; or we must let the eye look at one color through an inclined plate of glass beneath which it lies, whilst the upper surface of the glass reflects into the same eye another color placed alongside--the two lights then mix on the retina; or, finally, we must let the differently colored lights fall in succession upon the retina, so fast that the second is there before the impression made by the first has died away. This is best done by looking at a rapidly rotating disk whose sectors are of the several colors to be mixed.
Text from Project Gutenberg, public domain in the United States.