. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. movablevertical surfaces present in the eye-piece, serves as a micrometer; 4. Asight v, capable of being placed at different distances from the eye. Theflame impinges unchanged upon 0°, the sight and the microscope can turnhorizontally around the middle point from which the quadrant is described;the observed eye 0 is so placed, that its crystalline lens coincides with themiddle point of this quadrant. We can thus alternately fix the sight,placed near the eye, and a distant point s
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. movablevertical surfaces present in the eye-piece, serves as a micrometer; 4. Asight v, capable of being placed at different distances from the eye. Theflame impinges unchanged upon 0°, the sight and the microscope can turnhorizontally around the middle point from which the quadrant is described;the observed eye 0 is so placed, that its crystalline lens coincides with themiddle point of this quadrant. We can thus alternately fix the sight,placed near the eye, and a distant point situated in the direction of thesight, after having first given a proper position to the microscope and to thesight. The flame remains, as has been said, in the direction of 0°. If themicroscope be now placed at 60° and the sight close to 30°, we shall usually,on altering the accommodation, sec very distinctly the displacement and THE PHACOIDOSCOPE. 17 change of size of the middle image. Properly speaking, the sight oughtnot to stand at 30p, but at about 5° or 6° from it, as the observed eye must Fig. look 5° or 6° more inwards. The axis of vision lies, viz., in general, tothe outside of the line of vision (that is the line extending from the yellowspot to the fixed point), and the line of vision must therefore be turned in-wards, in order to direct the axis of vision to 30u. If we wish to determinethe position of the surface of reflexion of the lens from the distance betweenthe reflected images a and b, the direction of the axis of the cornea must firstbe sought in order to place the sight correctly with respect to it. Helmholtz constructed a peculiar instrument, called by him the ophthal-mometer, for the purpose of determining the magnitude of the reflected instrument is one of the great treasures for which we are indebted tohis genius. Kohlrausch, and also Benff, had already measured the reflected 18 CHANGE OF THE EYE IN ACCOMMODATION. images, at least
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