The Ophthalmoscope the Instrument Used to Check Eyes
The ophthalmoscope, the instrument used to look at the back of the eye through the pupil, allows the blood vessels to the surface of the retina. These blood vessels entering the eye through a circular region of the retina called decolorized the optical disc. It is also at this place, which is the head of the optic nerve, the ganglion cell axons converge to exit the eyeball.
Given the absence of photoreceptors in this area, the eye is insensitive to light, as indeed go where larger blood vessels. It is for this reason that the optical disc also receives the name of blind spot. Yet we do not feel any interruption in our visual field blind spot because the brain “compensates” somehow visual perception at these locations (see capsule experience left).
In the center of the retina is a darkest part, the macula, which is substantially devoid of blood vessels to optimize the central vision (as opposed to the peripheral vision). In the center of the macula, a small depression creates a black spot of about 2 mm diameter. This is the fovea, the point of the retina consists exclusively of cones where visual acuity is best.
Several vision problems can arise when light rays do not converge exactly on the retina.
If the eyeball is too short from front to back, for example, the rays converge beyond the retina. This defect called hyperopia is corrected by placing a convex lens in front of the eye lens that will accentuate the convergence and reduce the focus on the retina.
In the case of myopia, the convergence is done in front of the retina when the eyeball is too long, for example. Myopia by wearing concave lenses are corrected.
Another blurred vision, presbyopia, has a hardening of the lens related to age. This tightening reduces the elasticity of the lens, preventing him from taking a sufficiently rounded shape during accommodation and flatten enough during relaxation. The correction of presbyopia uses a bifocal lens, the upper portion being concave for distance vision and the lower convex portion for near vision.
