. A treatise on the diseases of the eye. The original form of Coccius ophthalmo-scope [Fig. 232] differed from that which I havedescribed above, and which is at present ingeneral use, both in being square in shape, andin being made of glass instead of metal. Thesquare mirror was inconvenient, and could notbe steadied so well against the orbit as thecircular. But the great disadvantage of the glassmirror was (as Helmholtz pointed out) thatthe aperture could not be bevelled down toso fine an edge as the metal one, in consequenceof which more or less of a canal existed, whichintercepted many of t
. A treatise on the diseases of the eye. The original form of Coccius ophthalmo-scope [Fig. 232] differed from that which I havedescribed above, and which is at present ingeneral use, both in being square in shape, andin being made of glass instead of metal. Thesquare mirror was inconvenient, and could notbe steadied so well against the orbit as thecircular. But the great disadvantage of the glassmirror was (as Helmholtz pointed out) thatthe aperture could not be bevelled down toso fine an edge as the metal one, in consequenceof which more or less of a canal existed, whichintercepted many of the peripheral rays, andproduced considerable diffraction. The mode of using Coccius ophthalmoscope is as follows : The collectinglens is to be turned towards the flame, which should be somewhat more thantwice the distance of the focal length of the lens from the observer. Themirror is then to be set somewhat slanting to the lens and the eye of thepatient. If the mirror is properly adjusted for the lens and the flame, weshall obtain, if
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Keywords: ., bookcentury1800, booksubjecteye, booksubjecteyediseases, bookyear
