. Elements of human physiology. Physiology. Diagram of reflected images from cornea and lens surfaces seen in phakoscope. a. From anterior surface of cornea, b. From anterior surface of lens. c. From posterior surface of lens. 1. During accommodation for distance. 2. During accommodation for near objects. Of a, b, and c the only measurement which presents any difHculty is b, the size of the image. For this purpose therefore the ophthalmometer was devised by Helmholtz. The principle of this instrument may be gathered from the diagram (Fig. 247). We may suppose that it is necessary to measure th
. Elements of human physiology. Physiology. Diagram of reflected images from cornea and lens surfaces seen in phakoscope. a. From anterior surface of cornea, b. From anterior surface of lens. c. From posterior surface of lens. 1. During accommodation for distance. 2. During accommodation for near objects. Of a, b, and c the only measurement which presents any difHculty is b, the size of the image. For this purpose therefore the ophthalmometer was devised by Helmholtz. The principle of this instrument may be gathered from the diagram (Fig. 247). We may suppose that it is necessary to measure the line a b, which may be taken to rejjresent an image reflected from the anterior surface of the cornea or lens. If we look at this line through a plate of glass the plane of which is at right angles to our line of sight, no distortion of the line a b takes place. If however the plate be placed obliquely, as at gr, </,, there will be an apparent shifting of the line sideways to c d. In the ophthalmometer there are two glass discs, ^, (/, and g^ g.,, one immediately over the other, so placed that the image a b is looked at through the junction between the two plates. The plates are then turned, as in the diagram, until a b appears as two distinct lines e c and c d just touching one another at c. At this point each image of the line a b has been shifted through one-half the length of a b. Knowing the thickness of the plates and their refractive index, it is easy to calculate, from the angle through which the plates have been turned, the apparent shifting of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Starling, Ernest Henry, 1866-1927. Chicago : W. T. Keener
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