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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    object viewed is at its focal length, we have simply to divide some distance taken as a standard by the focal length of the lens nsed. A distance of 8 inches has been agreed upon. The mag- nifying power therefore of a 2 inch lens = |- i= 4, of a 1 inch lens f = 8, of one-half inch lens f = 16, and so on. ]Now the focal length of the lenticular system of the eye has been calculated to be equal to 6.7 Paris lines, that is to say, the distance from the nodal point of the eye to the retina is 6.T lines. 8^' 96'^' The magnifying power of such a lens is consequently or ^ijjjf = 14^. The fundus of an ennnetropic eye is therefore seen under an enlargement of 14|- diameters. Moreover, when we look through a magnifying glass, placed close to our eye, at an object say, at its focal length, we do not see the object itself but its virtual image, and this image becomes, so to speak, for the time being, a defined picture, which the observer can project to any distance, finite or infinite, that he pleases. The greater the distance to which the image is projected, the greater the space which it appears to cover. Just as a small scotoma in one's eye may appear, when projected uj)on a piece of white paper held near the eye, to cover only a small circum- ference, but yet seem, when projected against the neighboring w^all, to occupy a large extent of surface. This is due, of course, merely to the increased opening of the visual angle. This may be illustrated in a very simple way, by imitating the condition of a normal eye. Set, for exam]3le, a one-inch lens so that it shall be just one inch from a piece of card on which some object—as a picture of the fundus, for instance—has been drawn. This is a rough but sufiiciently exact imitation of the eye.^ If we now place the model of the eye close to our own eye, we see an enlarged image of the picture beyond, which, by keeping the .other eye open, can be projected to any distance we see fit. So, ^ I might say here that a very convenient representation of the emmetropic eye can be had ready-made, in what is known in the shops as a cotton or linen counter. This consists of a small upright bit of brass, in which is set an inch lens of about half an inch in diameter. This upright is connected with a second upright by a short horizontal bottom piece which is just the focal length of the glass. To the second upright can be attached a bit of card with the picture of the fundus of the eye drawn upon it.