Lines for the detection of astigmatism. "The refracting surfaces of the eye acting together are equivalent in refracting power to a single, spherical surface of fairly short curvature. Frequently, however, the result is not the same as would be given by a perfect spherical surface, owing to inequalities in the curvature of the eye. In one direction the curvature may be greater than that at right angles to it. This tendency to a cylindrical form is called astigmatism. It interferes with the formation of perfect images and sometimes leads to serious eye strain in the effort to better the vision. Astigmatism may be detected by looking at black lines radiating from a point or at fine black concentric circles. Portions of the liens or circles appear gray and others black; the gray portions are out of focus. This defect is corrected by proper cylindrical glasses which equalize the curvature of the eye" — Newell, 1900

Detection of Astigmatism

Lines for the detection of astigmatism. "The refracting surfaces of the eye acting together are equivalent…

The convergence of light rays in the eyeball begins in the aqueous humor is perfected in the crystalline. But when they strike the vitreous humor, which is concave on its front surface, and also less dense than the crystalline, they diverse or spread out, and are thrown upon the back part of the eye.

Convergence of Rays in the Aqueous Humor of the Eyeball

The convergence of light rays in the eyeball begins in the aqueous humor is perfected in the crystalline.…

The organ of sight or vision.

Eye

The organ of sight or vision.

The human eye. Labels: a, crystalline lens; b, retina; c, cornea; d, sclerotic; e, choroid; g, ciliary processes; h, vitreous humor; k, pupil; i, iris; o, optic nerve.

Eye

The human eye. Labels: a, crystalline lens; b, retina; c, cornea; d, sclerotic; e, choroid; g, ciliary…

The eye and eyebrow of a person's face used for seeing.

Eye

The eye and eyebrow of a person's face used for seeing.

A human eye, used for sight, pictured with an eyebrow.

Eye

A human eye, used for sight, pictured with an eyebrow.

An illustration of an eye.

Eye

An illustration of an eye.

The eye and its muscles. Labels: o, the nerve of sight; a, one of the muscles of the eye.

The Eye and its Muscles

The eye and its muscles. Labels: o, the nerve of sight; a, one of the muscles of the eye.

Eye of the phalaena or butterfly, magnified, consisting of 11,300 square sections. The eye of the mordella is similar, containing 25,088 prisms.

Eye of the Butterfly

Eye of the phalaena or butterfly, magnified, consisting of 11,300 square sections. The eye of the mordella…

Eye of the yellow beetle magnified, composed of 8,820 hexagonal cylinders, the interior of each tube being round.

Eye of the Yellow Beetle

Eye of the yellow beetle magnified, composed of 8,820 hexagonal cylinders, the interior of each tube…

Plan of the eye seen in section. Labels: A, The Sclerotic Coat; B, The Choroid Coat; C, The Retina; D, The Optic Nerve; E, The Cornea; F, The Aqueous Humor; G, The Iris; H, The Ciliary Processes; I. The Crystalline Lens; K, The Vitreous Humor.

Diagram of the Eye

Plan of the eye seen in section. Labels: A, The Sclerotic Coat; B, The Choroid Coat; C, The Retina;…

Lens of the eye. The rays of light are brought nearer together by the lenses of the eye, just as they are by the lenses of the telescope.

Lens of the eye

Lens of the eye. The rays of light are brought nearer together by the lenses of the eye, just as they…

Section of the eye magnified, showing the ciliary processes, the pigmentum nigrum, the retina, and the choroid coat.

Section of the Eye

Section of the eye magnified, showing the ciliary processes, the pigmentum nigrum, the retina, and the…

Section of the eye magnified, showing the crystalline lens in its proper situation, between the aqueous and vitreous humors.

Section of the Eye

Section of the eye magnified, showing the crystalline lens in its proper situation, between the aqueous…

The vitreous humor and crystalline lens of the eyeball magnified, with the stains of the pigmentum nigrum left by the ciliary processes.

Section of the Eye

The vitreous humor and crystalline lens of the eyeball magnified, with the stains of the pigmentum nigrum…

The eye. Labels: a, sclerotica; e, cornea; b, choroid; d, optic nerve; f, aqueous humor; g g , iris; h, lens; i, vitreous humor; c, retina.

The Eye

The eye. Labels: a, sclerotica; e, cornea; b, choroid; d, optic nerve; f, aqueous humor; g g , iris;…

"The most essential parts of human vision are contained in the eyeball, a nearly spherical body, about an inch in diameter, and capable of being turned considerably in its socket by the action of various muscles." — Avery, 1895

Eyeball

"The most essential parts of human vision are contained in the eyeball, a nearly spherical body, about…

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctival membrane; 7, ciliary muscle; 10, choroid; 11, 13, ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery entering retina; 18, fovea centralis; 19, region where sensory part of retina ends; 26, 27, 28, are placed on the lens; 28 , suspensory ligament placed around lens; 29, vitreous humor; 30, aqueous humor in anterior chamber.

Left Eyeball in Horizontal Section

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic…

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctiva; 7, ciliary muscle; 10, choroid; 11, 13, ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery entering retina in optic nerve; 18, fovea centralis; 19, 20, region where sensory part of retina ends; 22, suspensory ligament; 24, the anterior part of the hyaloid membrane; 26, the lends; 29, vitreous humor; 30, aqueous humor.

The Eyeball in Horizontal Section

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic…

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic and cornea; 3, cornea; 4,5, conjunctiva; 6, posterior elastic layer of cornea; 7, ciliary muscle; 10, choroid; 11,13, ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery entering retina in optic nerve; 18, fovea centralis; 19, region where sensory part of retina ends; 22, suspensory ligament; 23 is placed in the canal of Petit, and the line from 25 points to it; 24, the anterior part of the hyaloid membrane; 26, 27, 28, are placed on the lends; 28 points to the line of attachment around it of the suspensory ligament; 29, vitreous humor; 30, anterior chamber of aqueous humor; 31, posterior chamber of aqueous humor.

Section of Left Eyeball

The left eyeball in horizontal section from before back. Labels: 1, sclerotic; 2, junction of sclerotic…

Front and side view of the eyeball.

Front and Side View of the Eyeball

Front and side view of the eyeball.

The eyeballs and their muscles as seen when the roof of the orbit has been moved and the fat in the cavity has been partly cleared away. On the right side the superior rectus muscle has been cut away. Labels: a, external rectus; s, superior rectus; i, internal rectus; t, superior oblique.

The Eyeballs and Their Muscles

The eyeballs and their muscles as seen when the roof of the orbit has been moved and the fat in the…

Diagram to illustrate the mechanism of accommodation (focusing); on the right half of the figure for a near, on the left for a distant, object. Labels: rf, ciliary muscle; ch, ciliary process of choroid; sl, suspensory ligament; i, iris.

Focusing of the Eye

Diagram to illustrate the mechanism of accommodation (focusing); on the right half of the figure for…

In passing through the crystalline, the rays cross each other, so that those rays which pass from the lower part of an object are presented uppermost in the bottom of the eye and the reverse, so that the images of object are always inverted, or bottom side upwards. Labels: a, the aqueous; c, the crystalline; v, the vitreous humor.

Formation of an Image on the Eyeball

In passing through the crystalline, the rays cross each other, so that those rays which pass from the…

Suppose a person was looking at a church with a tree standing at its side, he would have in each eye an actual inverted panorama of the objects, as seen in this image. The mind then reads this image as right-side up.

Formation of an Image on the Eyeball

Suppose a person was looking at a church with a tree standing at its side, he would have in each eye…

An illustration depicting the formation of circles of diffusion. "From point A luminous rays enter the eye in the form of a cone, the kind of which will depend on the pupil. Thus it may be circular, or oval, or ever triangular. If the pencil is focused in front of the retina, as at d, or behind it as it as at f, or, in other words, if the retina of being at F; be in the position G or H, there will be a luminous circle or a luminous triangular space, and many elements of the retina will be affected. The size of these diffusion circles depends on the distance from the retina of the point where the rays are focused: the greater the distance, the more extended will be the diffusion circle" (Britannica, 132).

Formation of Circles of Diffusion

An illustration depicting the formation of circles of diffusion. "From point A luminous rays enter the…

Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying debris. Although safety lenses may be constructed from a variety of materials of various impact resistance, certain standards suggest that they maintain a minimum 1 millimeter thickness at the thinnest point, regardless of material. Safety glasses can vary in the level of protection they provide.

Round Safety Glasses

Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying…

"...where we suppose that the object a, is brought within an inch or two of the eye, and that the rays proceeding from it enter the pupil so obliquely as not to be refracted b the lens, so as to form a distinct image." -Comstock 1850

Indistinct Vision

"...where we suppose that the object a, is brought within an inch or two of the eye, and that the rays…

Flexible crystalline lens of the eye.

Lens of the Eye

Flexible crystalline lens of the eye.

A convex lens, bends the ray of light which pass through it, so that they meet at a point called a focus. The crystalline lens converges the rays of light which enter the eye, and brings them to a focus on the retina. The healthy lens has a power of changing its convexity so as to adapt itself to near and to distant objects.

The Use of the Crystalline Lens

A convex lens, bends the ray of light which pass through it, so that they meet at a point called a focus.…

"Natural view of an arrow." -Comstock 1850

Natural Vision

"Natural view of an arrow." -Comstock 1850

The lenses and humors of the eye must be very exactly arranged, in order that the sight may be perfect. They must be so arranged that the images of the objects shall be formed distinctly on the retina. In nearsighted persons the lenses and humors are so arranged as to make the rays that form the images from together too quickly, before they reach the retina, as shown in the figure.

Nearsighted Vision

The lenses and humors of the eye must be very exactly arranged, in order that the sight may be perfect.…

View of the second pair of optic nerves.

Optic Nerves

View of the second pair of optic nerves.

View of the second pair of optic nerves.

Optic Nerves

View of the second pair of optic nerves.

"This picture is based upon a photograph of a painting which has won great fame during recent years. The artist has represented Pasteur, the celebrated scientist, busily at work in his laboratory, recording the results of his microscopal study of bacteria. As the great master of modern science rests for a moment from his midnight researches, he appears to see in a vision the victims of pestilence, famine, and infectious diseases imploring relief from the good angel who symbolizes the wonderful victories won by Pasteur in combating disease." — Blaisedell, 1904

Louis Pasteur

"This picture is based upon a photograph of a painting which has won great fame during recent years.…

On the left (a) the pupil is wide open (dilated), while on the right (b) the pupil is contracted. The iris regulates the opening and closing of the pupil and it has two sets of fibers, straight and circular. When the pupil is open the circular fibers of the iris are relaxed while the straight ones are open. When the pupil is contracted, the straight fibers of the iris are relaxed while the circular ones are contracted.

Pupil Contraction and Dilation

On the left (a) the pupil is wide open (dilated), while on the right (b) the pupil is contracted. The…

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor were removed.

The Retina

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor…

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.

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor were removed. The white disk to the right marks the entry of the optic nerve (blind spot); the lines radiating from this are the retinal arteries and veins. The small central dark patch is the yellow spot, the region of most acute vision.

Retina Blind Spot

The right retina as it would be seen if the front part of the eyeball with the lens and vitreous humor…

A section through the retina from it anterior inner surface (1) in contact with the hyaloid membrane, to its outer (10) in contact with the choroid. 1, internal limiting membrane; 2, nerve-fiber layer; 3, nerve cell layer; 4, inner molecular layer; 5, inner granular layer; 6, outer molecular layer; 7, outer granular layer; 8, external limiting membrane; 9, rod and cone layer; 10, pigment cell layer.

Section of Retina

A section through the retina from it anterior inner surface (1) in contact with the hyaloid membrane,…

Formation of image on the retina.

Formation of Image on the Regina

Formation of image on the retina.

Formation of an image in the retina.

Image Formation in the Retina

Formation of an image in the retina.

Diagram showing relations of the neurons and sensory epithelium in the retina. labels: E, epithelial layer of nucleated rode and cone cells, rods being directed towards choroid coat of retina; N1, neurons of first series receiving by their dendritic impulses from the rod and cone cells and transmitting them by their axons to N2, the neurons of the second series. The axons of the neurons of the second series pass along the inner surface of the retina to the blind spot, where they unite to form the optic nerve.

Neurons and Sensory Epithelium in the Retina

Diagram showing relations of the neurons and sensory epithelium in the retina. labels: E, epithelial…

Structure of the human retina, showing the various layers.

Structure of the Retina

Structure of the human retina, showing the various layers.

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 the structure of the human retina. Labels: I, pigment layer; II, rod and cone layer; R, rods; C, cones; III-IX, intraretinal nerve-elements; X, axons which pass to optic nerve.

Retinal Structure

Diagram of the structure of the human retina. Labels: I, pigment layer; II, rod and cone layer; R, rods;…

The last appearance of the sangreal.

Sangreal

The last appearance of the sangreal.

A. a shortsighted eye; B. an arrow which it attempts to perceive, but is prevented by the convergence of the passage of the visual rays to foci, at C., before they reach the retina at D. E. the same eye similarly situated, showing how by the interventions of a concave lens, G., the rays are diverged, and the image of the arrow, F., accurately converged to the retina at A.

Shortsighted Vision and Correction

A. a shortsighted eye; B. an arrow which it attempts to perceive, but is prevented by the convergence…

"The stereoscope is an instrument for illustrating the phenomena of binocular vision, and for producing from two nearly similar pictures of an object the effect of a single picture with the appearance of relief and solidity that pertains to ordinary vision." — Avery, 1895

Stereoscope

"The stereoscope is an instrument for illustrating the phenomena of binocular vision, and for producing…

"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."…

An illustration of a transverse section of an ideal eye. "A, summit of cornea; SC, sclerotic; S, Schlemm's canal; CH, choroid; I, iris; M, ciliary muscle; R, retina; N, optic nerve; HA, aqueous humour; L, crystalline lens, the anterior of the double lines on its face showing its form during accommodation; HV, vitreous humour; DN, internal rectus muscle YY', principle optical axis; C, [center] of the ocular globe..." (Britannica, 132).

Transverse Section of an Ideal Eye

An illustration of a transverse section of an ideal eye. "A, summit of cornea; SC, sclerotic; S, Schlemm's…

Diagram illustrating rays of light converging in a normal eye (A), a myopic eye (B), and a hypermetropic eye (C)

Vision

Diagram illustrating rays of light converging in a normal eye (A), a myopic eye (B), and a hypermetropic…

The vision of the explorer Christopher Columbus.

The Vision of Columbus

The vision of the explorer Christopher Columbus.