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Head of an eagle owl. It has large eyes for seeing at night, and a hooked beak for snaring its prey.

Head of an Eagle Owl

Head of an eagle owl. It has large eyes for seeing at night, and a hooked beak for snaring its prey.

"Pearly Nautilus. e, eye; h, hood, a muscular portion of the foot which protects the softer parts; s, siphon; se, septa, separating the successive chambers of the shell; sp, siphuncle; t, tentacles." — Galloway

Pearly Nautilus

"Pearly Nautilus. e, eye; h, hood, a muscular portion of the foot which protects the softer parts; s,…

"A representation of the manner in which the image is formed upon the retina in the perfect eye. The curvature of the cornea, s s, and of the crystalline lens, c c, is just sufficient to cause the rays of light proceeding from the image, e e, to converge the right focus, m, upon the retina." —Wells, 1857

Perfect Eye

"A representation of the manner in which the image is formed upon the retina in the perfect eye. The…

Pigment epithelium of human retina (viewed from the surface).

Pigment Epithelium of Retina

Pigment epithelium of human retina (viewed from the surface).

The pochard (or dun-hen) averages about nineteen and a half inches in length, and is found in parts of Europe. In England it is sometimes known as the red-headed poker and red-eyed poker.

Pochard

The pochard (or dun-hen) averages about nineteen and a half inches in length, and is found in parts…

The external muscles of the pupil.

External Muscles of the Pupil

The external muscles of the pupil.

Vertical section of the pupil.

Vertical Section of the Pupil

Vertical section of the pupil.

"Showing the attachment of the recti, or straight muscles to the eyeball, also the distribution of arteries upon the outer coat of the eye." — Blaisedell, 1904

Attachment of the recti

"Showing the attachment of the recti, or straight muscles to the eyeball, also the distribution of arteries…

"Is a very common fish in Europe; it is of brilliant colors, and very tenacious of life; it feeds on worms, molluscuous animals, insects, and some vegetable matters." — Goodrich, 1859

Redeye

"Is a very common fish in Europe; it is of brilliant colors, and very tenacious of life; it feeds on…

"The following simple experiment illustrates the effect of refraction:—Place a silver coin, m, at the bottom of the basin. The rays, i i, proceeding to the eye from the silver surface, render the coin visible. The point a, the eye, is then moved farther bck, so that the edge of the basin obstructs the direct rays, and of course the coin is no longer seen. If an attendant carefully pours water into the basin, so that the object is not moved, it will presently, as the water rises in the basin, become again visible. This arises from the refraction of the rays by the water, the image, indeed, appearing at n instead of at m." —Wells, 1857

Refraction

"The following simple experiment illustrates the effect of refraction:—Place a silver coin, m,…

"Illustration depicting the refraction by the eye." -Comstock 1850

Refraction by the Eye

"Illustration depicting the refraction by the eye." -Comstock 1850

A diagram showing how an image is formed upon the retina by the crystalline lens.

A Diagram of the Retina and Crystalline Lens

A diagram showing how an image is formed upon the retina by the crystalline lens.

A flattened shape of the globe of the eye causes farsightedness and an elongated shape of the globe of the eye causes nearsightedness. The diagram illustrates the position of the retina in these conditions. Labels: B, natural sight; G, farsightedness; C, nearsightedness.

The Position of the Retina in Near and Far Sight

A flattened shape of the globe of the eye causes farsightedness and an elongated shape of the globe…

Diagrammatic section of the human retina.

Section of Retina

Diagrammatic section of the human retina.

Perpendicular section of mammalian retina. Labels: A, layer of rods and cones; B, outer nuclear layer; C, outer molecular layer; D, inner nuclear layer; E, inner molecular layer; F, ganglionic layer; G, stratum opticum; r, rods; c, cones, r.g, rod granules; c.g, cone granules; r.b, rod bipolars; c.b, cone bipolars; c.r, vontact of rod bipolars with the internal arborisation of cone bipolars; ar', internal arborisation of rod bipolars; c.n, centrifugal nerve fiber; h, horizontal cells; s.s, stratified spongioblasts; d.s, diffuse spongioblasts; s.g, stratified ganglion cell; M, sustentacular fiber of Muller.

Section of Retina

Perpendicular section of mammalian retina. Labels: A, layer of rods and cones; B, outer nuclear layer;…

Next to the choroid and comprising about 1/4 the entire thickness of the retina is a multitude of transparent, colorless, microscopic rods (a) evenly arranged and packed side by side, like the seeds in the disk of a sunflower. Among them at regular intervals, are interspersed the cones (b). Delicate nerve fibers pass from the ends of the rods and cones, each expanding into a granular body (c) thence weaving a mesh (d) and again expanding into the granules (f). Last is a layer of fine nerve fibers (g) and gray, ganglionic cells (h) like the gray matter of the brain, whence filaments extend into the fibers of the optic nerve (i).

The Structure of the Retina

Next to the choroid and comprising about 1/4 the entire thickness of the retina is a multitude of transparent,…

"Diagram of a retinula of the central eye of a scorpion consisting of five retina-cells (ret), with adherent branched pigment cells (pig)." — The Encyclopedia Britannica, 1910

Retinula

"Diagram of a retinula of the central eye of a scorpion consisting of five retina-cells (ret), with…

"Rhabdom of the same, consisting of five confluent rhabdomeres." — The Encyclopedia Britannica, 1910

Rhabdom

"Rhabdom of the same, consisting of five confluent rhabdomeres." — The Encyclopedia Britannica,…

"Transverse section of the rhabdom of a retinula of the scorpion's central eye, showing its five constituent rhabdomeres as rays of a star." — The Encyclopedia Britannica, 1910

Rhabdom

"Transverse section of the rhabdom of a retinula of the scorpion's central eye, showing its five constituent…

"Development of the lateral eyes of a scorpion. h, Epidermic cell-layer; mes, mesoblastic connective tissue; n, nerves; II, III, IV, V, depressions of the epidermis in each of which a cuticular lens will be formed." — The Encyclopedia Britannica, 1910

Scorpion Eyes

"Development of the lateral eyes of a scorpion. h, Epidermic cell-layer; mes, mesoblastic connective…

The tear gland of the eye. The tears are carried from this factory b little ducts (b) and are poured over the surface of the eye. They serve to keep the eye moist, so that it can be moved about in its socket easily by the muscles. They also serve to wash out substances that get into the eye. The tears go into openings (c c) and pass through two ducts which unite in one (d e) which ends in the nose.

Tear Gland

The tear gland of the eye. The tears are carried from this factory b little ducts (b) and are poured…

"A telescope is an instrument designed for the observation of distant objects, and consists essentially of an objective for the formation of an image of the object and of an eyepiece for magnifying this image." — Avery, 1895

Telescope

"A telescope is an instrument designed for the observation of distant objects, and consists essentially…

"The eye-end of a telescope. The reader will recognize the micrometer previously described. L is a paraffin lamp fitting by a bayonet joint into a copper cover c. This effectually defends its glass chimney against accident, and protects the lamp from wind. The simple means by which this lamp is made to preserve its verticality in all positions of the telescope is evident from the figure. By this lamp alone the bright wire or bright field illumination is given at pleasure, and with any desired intensity, simply by movement of the small pin p." —The Encyclopedia Britannica, 1903

Telescope

"The eye-end of a telescope. The reader will recognize the micrometer previously described. L is a paraffin…

"The Galilean telescope has a double-concave eye-lens that intercepts the rays before they reach the focus of the objective." — Avery, 1895

Galilean Telescope

"The Galilean telescope has a double-concave eye-lens that intercepts the rays before they reach the…

"The reflecting telescope has an objective a concave mirror, tecnically called a speculum. The images formed by the speculum are brought to the eyepiece in several different ways. Sometimes the eyepiece consists of a series of convex lenses placed in a horizontal tube." — Avery, 1895

Reflecting Telescope

"The reflecting telescope has an objective a concave mirror, tecnically called a speculum. The images…

"The spy-glass or terrestrial telescope avoids the inversion of the image by the interposition of two double-convex lenses, m and n, between the objective and eyepiece. The rays diverging from the inverted image at I cross between m and n, and form an erect magnified, virtual image at ab." — Avery, 1895

Terrestrial Telescope

"The spy-glass or terrestrial telescope avoids the inversion of the image by the interposition of two…

"The Actual Size of the Test Type, which shiykd be seen by the Normal Eye at a distance of Twenty Feet." — Blaisedell, 1904

Test type

"The Actual Size of the Test Type, which shiykd be seen by the Normal Eye at a distance of Twenty Feet."…

"The workmen all their master will obey, / They plane and saw, and dare not stop to play; / Each boy and man the master keeps in view, / His eye does more than both his hands can do."—barber, 1857

The Eye of the Master Does More Work than Both His Hands

"The workmen all their master will obey, / They plane and saw, and dare not stop to play; / Each boy…

"Diagrams illustrating two stages in the development of the vertebrate eye. A, showing the relation of the ectoderm, the brain vesicle, and the optic vesicle. The right side of the figure shows a later stage than the left. B, later stage, showing the lens, eye-ball and retina in position. b.v., brain vesicle formed by the invagination oof the ectoderm (ect.); l, lens; mes., mesodermal tissue; o.n., optic nerve; o.s., optic stalk; o.v., optic vesicle, a portion of the brain vesicle; r, retinal layer; v.h., interior of eye-ball which comes to contain the vitreous humor." — Galloway

Vertebrate Eye

"Diagrams illustrating two stages in the development of the vertebrate eye. A, showing the relation…