Page:Handbook of Ophthalmology (3rd edition).djvu/21

eye, we must examine it more closely, and apply to it the formula for conjugate foci.

Fig. 3.

If, for instance, the convex lens in Fig. 3 have a focal length of 3 inches, then ${\frac {1}{f}}={\frac {1}{3}}$; and if the screen be 2 inches from the lens, then ${\frac {1}{a}}={\frac {1}{2}}$, and we have from the formula

${\frac {1}{a}}+{\frac {1}{\alpha }}={\frac {1}{f}}$
${\frac {1}{2}}+{\frac {1}{\alpha }}={\frac {1}{3}}$, or ${\frac {1}{\alpha }}={\frac {1}{3}}-{\frac {1}{2}}=-{\frac {1}{6}}$. The negative sign shows that rays of light which proceed from $a$, after refraction in the convex lens, diverge as if they had proceeded from $\alpha$; $\alpha$ is the image-point of $a$; but since the rays do not actually intersect at $\alpha$, but only diverge as if they had proceeded from it, the image at $\alpha$ is called a virtual image.

It follows further that when we can give to rays proceeding from any given point a direction such that they converge toward the point $\alpha$, they will after refraction in the convex lens form an optical image at the point $a$. Under these conditions a distinct image will be formed in spite of the faulty construction of the camera obscura; these conditions must be fulfilled in the case of the hypermetropic eye.

EMMETROPIA.

According to the foregoing the optical construction of the emmetropic eye is such that with absolute relaxation of accommodation it is adjusted for far-distant objects, and throws distinct retinal images of them. The ability to see near objects distinctly depends upon the accommodation.

The limits of accommodation are called respectively the far and the near point. The position of the far point depends upon the condition of refraction. It is the most distant luminous point whose rays can still be united in an image upon the retina. Assuming the accommodation to be relaxed, the far point of the emmetropic eye lies at an infinite distance, since light which proceeds