A practical treatise on diseases of the eye / by Robert Brudenell Carter.

  • Robert Brudenell Carter
Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: A practical treatise on diseases of the eye / by Robert Brudenell Carter. Source: Wellcome Collection.

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    threads or filaments variously curved and disposed. They are most conspicuous when the eyes are turned towards a white sur- face, sucli as a whitewashed wall or ceiling, or a white cloud; and it is characteristic of them that they never cross the axis of vision, nor obscure or conceal the spot which is looked at, but move about over lateral portions of the field. They are due to the shadows cast upon the retina by the minute objects in the vitreous body which have been described at page 29; and these shadows are more visible to myopic persons than to others, because in the former the greater length of the vitreous chamber allows a given object to cast upon the retina a shadow of larger size. A temporary increase of muscee may be oc- casioned by any conditions which disturb the balance of the ocular circulation, and thus alter the density and the index of refraction of the fluids within the eye. Within certain limits they must be regarded as physiological phenomena, which require only to be neglected by the patient; but, when they are either very abundant or steadily increasing, they may point to actual or impending morbid changes of a serious character; and they should lead, under such conditions, to a careful examina- tion of the eyes, and also of the general state of the patient. The materials of which spectacle lenses are made are two in number: flint glass, and fluor spar, or, as it is commonly called, pebble. In former times, when the manufacture of glass was less perfectly conducted than at present, this material was liable to undergo chemical changes which diminished its transparency ; so that glass lenses, after a certain period of wear, became dull and clouded. At that time, ' pebbles ' were held in high esteem; but, now that good glass is as permanently transparent as pebble itself, the latter no longer possesses the advantages which were once justly claimed for it. Pebble is still, however, harder than glass, so as to be less liable to scratches or accidental fracture; and it is also, for the same refractive power, less heavy and less bulky. Hence it is to be preferred, when the somewhat greater cost is not an obstacle, for convex lenses generally, for all lenses of high power, and for such as are likely to be worn for a long time without variation; whenever, in short, durability and lightness are important. For concave lenses, the recessed surfaces of which are little liable to injury, and for convex lenses used only p p 2
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