Text-book of ophthalmology . TION OF THE EYES 85 the other hand, as in keratoconus, the cornea is unduly yielding, the intra-ocularpressure may be abnormally high, and yet the tension shown by the tonometermay be normal or subnormal. In this case there is said to be a relative increaseof tension (Strebel and Steiger). And even if the intra-ocular pressure itself is nohigher than usual, it must be regarded as relatively too high, if, as in keratoconusand progressive myopia, the ocular envelopes are so yielding as to give waybefore it.—D.] Examination with the Ophthalmoscope (Ophthalmoscopy) 82.
Text-book of ophthalmology . TION OF THE EYES 85 the other hand, as in keratoconus, the cornea is unduly yielding, the intra-ocularpressure may be abnormally high, and yet the tension shown by the tonometermay be normal or subnormal. In this case there is said to be a relative increaseof tension (Strebel and Steiger). And even if the intra-ocular pressure itself is nohigher than usual, it must be regarded as relatively too high, if, as in keratoconusand progressive myopia, the ocular envelopes are so yielding as to give waybefore it.—D.] Examination with the Ophthalmoscope (Ophthalmoscopy) 82. The invention of the ophthalmoscope by Helmholtz in the year1851 was one of the most beneficent achievements in modern medicine. Ithas made the interior of the eye accessible to investigation; blood-vesselsand nerves, which in the rest of the body are exposed only by surgical man-ipulation, here lie unveiled before us and permit us to study their minutestvariations. In ophthalmology, the ophthalmoscope has produced a com-. P Fig. 9.—Principle of Helmholtzs Ophthalmoscope. plete revolution, since it has thrown light into the dark region of what wasformerly called black cataract, and has acquainted us with the manifoldmorbid processes which lie at the root of this dreaded malady. Manyof these processes, if diagnosticated correctly and in time, would, at the pres-ent day, receive successful treatment. Furthermore, in general medi-cine the ophthalmoscope has become an indispensable aid to diagnosis,since many internal disorders produce characteristic changes in the fun-dus of the eye. Principle of the Ophthalmoscope.—In order to see the fundus of aneye, we must throw light by the aid of suitable apparatus through the pupiland upon the fundus, and receive the light reflected from the latter into ourown eye and unite the rays to form a sharp image. In the original ophthal-moscope of Helmholtz this end was obtained in the following way: Beforethe eye under investigation (A, Fig
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