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; STENOPyEIC SLIT. (% Size) 



EFFECT OF THE A Black metal discs of the size of trial lenses, one 



UPIL - with a pupillary aperture of 2.5 mm. and the other 



of 7.5*mm. 



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 stenopasic slit. 



