Page:Handbook of Ophthalmology (3rd edition).djvu/37

MYOPIA. '31

Myopia exists when, accommodation being relaxed, parallel rays falling upon the cornea are not united to an image-point upon the retina, but intersect each other in front of it. Of course under these circumstances every distant object casts upon the retina a circle of diiFusion. The retinal images of distant objects are there- fore diifuse and indistinct. In order to cast a sharp image upon the retina the luminous point or object fixed must approach the eye to within a certain distance. The far point of distinct vision does not lie as M'ith the emmetropic eye at an infinite but at a finite distance.

The position of the far point can be directly determined by ascertaining the greatest distance at which test type of the proper size can be read. If, for instance, the patient read No. I of Snellen's test type at 12 inches, but cannot read No. II at 24 inches, it may be assumed with certainty that his far point lies between 12 and 24 inches, and it remains, by further experiments, to determine more exactly its position.

It is advisable in all cases to begin the examination in this manner and then to verify and perfect the result by giving to the rays from a distant object, by the help of a concave lens, a divergence such as if they had proceeded from the far point. If, for instance, the far point lie at 16 inches, and we place imme- diately in front of the patient's eye a concave lens with a negative focal length of 16 inches, then will the parallel rays after their refraction in the concave lens so diverge as if they had proceeded from the negative focal point of the lens ; but according to our supposition this negative focal point coincides with the for point ; both lie 16 inches from the eye, the distance between the concave lens and the eye not being considered. Under these circumstances there is distinct distant vision, and upon using the test type for dis- tance, normal acuteness of vision is found. A concave lens weaker than — yig would not produce this result, for its negative focal ])oint would lie at a greater distance from the eye than the far point ; but with a stronger lens, for instance with — -^^ or — ^^, dis- tant vision is still possible; the rays falling parallel upon the lens diverge after their refraction in it, as if they had proceeded from a point 14 or 12 inches distant, and a myopic eye whose far point lies 16 inches distant, can by a slight effort of accommodation