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CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 669
addition to its magnifying or reducing power. Acting as a cylinder it follows the law of the cylinder as given in § 929.
If one keeps in mind the effect of oblique lenses it will help to appreciate the necessity of having the spectacles properly adjusted.
§ 931. Effect of the aperture of the pupil in vision. — As a
general statement it may be said that the larger the aperture of the pupil the more brilliant will be the image as more light is admitted. On the other hand the larger the pupil the more strongly do eye defects deteriorate the retinal image.
When the aperture of the pupil is small, only a small part of the refracting surface produces the image, and consequently any defects of curvature are minimized ; but the small aperture makes the image less brilliant as only a limited amount of light goes to form it, and furthermore it requires muscular effort to contract the iris to make the pupil small. With a small pupil, objects can be seen clearly only when they are in a brilliant light, hence eye defects cannot be compensated for in a dimly lighted place by closing the pupil.
For demonstrating the effect of the pupillary aperture there are needed :
FIG. 398. TRIAL LEN FIG. 399. Discs WITH PUPILS OF LARGE AND SMALL
SES TO SHOW THE APERTURE; STENOP/EIC SLIT. (% size)
EFFECT OF THE A Black metal discs of the size of trial lenses, one
with a pupillary aperture of 2.5 mm. and the other
B Stenopaeic slit.
(1) A 3 or 4 diopter convex projection trial lens (fig. 398).
(2) A 0.5 diopter concave or convex cylinder.
(3) Two black discs the size of trial lenses and with apertures, one of 2.5 mm., and one of 7.5 mm.
(4) A lantern slide of the radial lines.
(5) A black disc with a Stenopaeic slit.