A text-book of physiology for medical students and physicians . of thecorneal images may be measured for the different meridians and thus enableone to compare their curvatures. In modern instruments, such as is repre-sented in Fig. 140, the luminous areas, known as targets or mires, are placed in aspherical shield which may be rotated around the axis of the telescope. Theshield has a radius of curvature of meters and its center of rotation isapproximately coincident with that of the cornea when the eye is in itsproper position. The reflected images of the mires from the surface of the DI
A text-book of physiology for medical students and physicians . of thecorneal images may be measured for the different meridians and thus enableone to compare their curvatures. In modern instruments, such as is repre-sented in Fig. 140, the luminous areas, known as targets or mires, are placed in aspherical shield which may be rotated around the axis of the telescope. Theshield has a radius of curvature of meters and its center of rotation isapproximately coincident with that of the cornea when the eye is in itsproper position. The reflected images of the mires from the surface of the DIOPTRICS OF THE EYE. 329 cornea are each doubled, when viewed through the telescope, by means ofa double vision prism of Iceland spar and the displacement produced inthis way is a definite amount for the distance chosen. _ Four images of themires are thus seen, and when the mires are properly adjusted for a cornea ofaverage curvature the two inner images are in contact with each variation from this average is indicated by an overlapping of the images,. Fig. 140.—Ophthalmometer (Hardy . the value of which in diopters or in radii of curvature is read off upon theinstrument. The instrument, therefore, when once calibrated enables oneto read off at once the radii of curvature for the different meridians and thusdetermine the axis and degree of astigmatism. It should be added thatthe instrument gives only the curvatures and degree of astigmatism, if anyexists, of the cornea, and is therefore of no immediate service in determiningthe total astigmatism, that is, the astigmatism of cornea and lens actingtogether. CHAPTER XVIII. THE PROPERTIES OF THE RETINA—VISUAL STIMULIAND VISUAL SENSATIONS. The Portion of the Retina Stimulated by Light.—The normalstimulus to the sensory cells in the retina is found in the vibrationsof the ether, the waves of light. When sunlight is passed through aprism the waves of different lengths are dispersed, and those capableof stimulating the ret
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