Farmasevtika instituti farmakologiya va klinik farmatsiya kafedrasi

 
 
Formation of an Image on the Retina. 
In the same manner that a glass lens can focus an 
image on a sheet of paper, the lens system of the eye can focus an image on the retina. The image is 
inverted and reversed with respect to the object. However, the mind perceives objects in the upright 
position despite the upside-down orientation on the retina because the brain is trained to consider an 
inverted image as normal. 
Mechanism of ―Accommodation‖ 
In children, the refractive power of the lens of the eye can 
be increased voluntarily from 20 diopters to about34 diopters; this in an ―accommodation‖ of 14 
diopters.To do this, the shape of the lens is changed from that of a moderately convex lens to that of a 
very convex lens. The mechanism is as follows. In a young person, the lens is composed of a strong
lastic capsule filled with viscous, proteinaceous, but transparent fluid.
 
Pupillary Diameter 
The major function of the iris is to increase the amount of light that enters 
the eye during darkness and to decrease the amount of light that enters the eye in daylight.
Figure 
49-10 
Mechanism of accommodation (focusing).
Unit X The Nervous System: B. The Special 
Senses 
602 
The amount of light that enters the eye through the pupil is proportional to the 
area 
of 
the pupil or to the 
square of the diameter 
of the pupil.
Figure 49-11 
Effect of small (
top
) and 
large (
bottom
) papillary apertures on “depth of focus.” 
Emmetropia Hyperopia Myopia 
Figure 
49-12 
Parallel light rays focus on the retina in emmetropia, behind the retina in hyperopia, 
and in front of the retina in myopia. Chapter 49 The Eye: I. Optics of Vision 603 
the person can 
use the mechanism of accommodation to keep the image focused clearly. A myopic person has a 
definite limiting ―far point‖ for clear vision.