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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    As the accommodation is equivalent to a convex glass of differ- ent focal lengths, it is evident that the observer may substitute his own accommodation for the glass, provided he knows j ast how much he is using, and how far his nodal point is from that of the examined eye. For example, if the observer sees an eye distinctly, while he is conscious that he is accommodating for ten inches, he knows that the Hm the observed eye must be equal to yV minus the distance between the nodal points of his own and the observed eye. If this is two inches, then J_ _ 2 = ■§-. The ability to judge of refraction by the degi-ee of tension re- quired of the accommodation, can only of course be brought into play in one condition, that is, where the observed eye is hyper- metropic, and even here it is rather a tour deforce than an essen- tial advantage. We can all of us by a little practice get an ap- proximate idea as to the amount of hypermetropia in a given case, by the amount of tension required of our accommodation in get- ting a clear view of the fundus, but very few even with any amount of practice ever approximate that precision which can be obtained with infinitely less trouble by means of glasses. As in the above cases the rays of light passing thi-ough the hole of the mirror are parallel, and wdll continue so if uninterrupted to infinity, it makes no difference in the result whether the ob- server's eye is close against the instrument or a little removed from it. The only calculation necessary is the distance between the glass and nodal point of the examined eye. The above directions, wdiich are sufficient for an emmetropic observer whose eye is in a state of rest to determine any condition of refraction, may be summed up in this general rule :— The cmnetro^ia in a given case is equal to the glass 'used ])l%is the distance between it and the nodal jpoint if the eye examined he m.yojpic^ minus the distance if it he hyjpermetrojpic. If, however, the observer is so unfortunate as to be ametropic, then the simplest w^forhim^is^ to reduce his eye to a condition cf emmetropia, that is to say, to that condition of refraction that parallel rays unite on his i-etina; considering that portion of the accommodation which cannot be relaxed as part and parcel of the refraction. If the ametropic observer does this, then of course the preced-