Spherical aberration.
A lens having a spherical surface bends the rays so that they do not all come to a focus at the same point. What is the effect of this on our theoretically perfect image? Again it is slight blurring of the sharpness of outline. It is said the spherical aberration in a perfectly corrected optician’s lens is less than that in the lens of the human eye. This must be remembered in connection with our later remarks. In the lower animals, spherical aberration is nearly absent. Their vision therefore is more periscopic, and therefore more like that of an optician’s lens.
Astigmatism.
This defect can be avoided in the optician’s lens, but it exists in, and is a serious fault of, the human eye.
Helmholtz considers the amount of spherical aberration unimportant as compared with this defect. Astigmatism is the result of imperfect symmetrical curvature of the cornea and of imperfect centering of the cornea and lens. This defect is found in most human eyes.
Astigmatism prevents the eye seeing vertical and horizontal lines at the same distance perfectly clearly at once. The defect in centering also causes irregular radiation, so that, as Helmholtz says, “The images of an illuminated point as the human eye brings them to focus, are inaccurate.” What is the effect of those defects on the “perfect image”? Dimness of outline and detail in the textures of objects seen.
Turbidity.
The optician’s lens is made of pure glass, the media of the human eye are not clear, but slightly turbid, so that Helmholtz says, “The obscurity of dark objects when seen near very bright ones depends essentially on this defect. This defect is most apparent in the blue and violet rays of the solar spectrum; for then comes in the phenomena of fluorescence to increase it.” |Fluorescence.| By fluorescence is meant the property which certain minutely divided substances possess of becoming faintly luminous, so long as they receive violet and blue light. The bottles filled with solution containing quinine, which look blue in the chemists' windows, owe their colour to this fact, as also does the blueness of “London” milk. These defects, combined with entoptic impurities which are constantly floating about in the humours, all help to detract from the brightness and sharpness of the “perfect image.”
Blind spot.
This is a portion of the retinal field with no cones or rods, and therefore insensitive to light. This causes a gap in the field of vision. “This blind spot is so large that it might prevent our seeing eleven full moons if placed side by side, or a man’s face at a distance of only six or seven feet,” says Helmholtz. In addition to this, there are lesser gaps in the retinal field, due to the cutting off of light by the shadows thrown by the blood vessels. Any one who has examined the retinal field with an ophthalmoscope knows what this means.