Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring.

  • Loring, Edward G. (Edward Greeley), 1837-1888.
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    Tlie intervals between the focal leno^ths of these 2-lasses are small enough to fulfil every requirement in ophthalmoscopy, with- out the perplexing resort of adding and subtracting vulgar frac- tions with comparatively high denominators. If required, how- ever, it is very easy to vary the focal interval in all the stronger, and in many of the moderately strong glasses, that is, from \ to Y^y. This is done by the observer simply withdrawing his head half an inch from the ordinary place of examination. If, for example, it is found that in the usual place — ^ in a given case is too strong, the head has only to be withdrawn one-half inch, and the glass has then the eifect of ~. With -J it becomes ~^ and so on through the series. So, too, with the convex glasses ; only here the glasses increase in strength as we withdraw the head. Thus if we find that + i is not quite strong enough in a given case, we have only to withdraw our head half an inch and the o^lass has the same effect as ~p would have had if the head had not been moved. Thus, with little or no inconvenience, a large num- ber of additional focal values, with small intervals of refraction, can be obtained. The disk is divided perpendicularly into two equal parts, one of which contains the convex, the other the con- cave. In each half the weaker and more commonly occurring glasses are set in the outer semicircle, while the stronger ones are put in the inner one. The outer or inner circle can be rapidly brought into position by simply sliding the disk upwards or downwards on the handle by means of the thumb of the hand which holds the instrument, and this can be done if desirable w^ithout removing it from the eye. By this simple contrivance all the necessary glasses are con- tained in a single stationary but re\olving disk, and they can be brought into position with the least possible delay or inconveni- ence. In order to clean the glasses, which need only be done very rarely, it is only necessary to unscrew the handle near the mirror, and the disk can then be readily slipped off the han- dle and both surfaces of the glasses thus exposed. The mirror, being contained in a separate cell, is made detach- able, so that a weak light mirror can be substituted, or the instru- ment be used, when the mirror is remo\ ed, as an optometer. While every effort has been made to make the parts necessary