An additional method to determine the degree of ametropia.

  • Thomson, William, 1833-1907.
Date:
[1871]
Catalogue details

Licence: Public Domain Mark

Credit: An additional method to determine the degree of ametropia. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    3/6 (page 5)
    convergence of the rays from the nodal point. The myopia was then found to be corrected perfectly by a —J placed J from the nodal point. In a case of aphakia, after a successful operation for cataract, the distance between the double lights was found to be 10 inches, or 250 mm.; and X — = 80 mm., or 3.2 inches. 250 This case required for its correction a + J.5, J from the no- dal point, showing that the hypermetropia = A case of hypermetropic astigmatism in a person able to give concise replies, and who could fully relax his accommoda- tion, gave the following results. With the holes so placed as to analyze one meridian, the distance apart was 4 = 101 mm.; and in the meridian at right angles to first, 4.5, or 115 mm. Let x be the degree of ametropia, and we have the formula: x = = 196 or .7.7 in., which indicates that the rays 101 diverge from this meridian as though they arose from a point 7.7 behind its nodal point, and that a convex |.2 would suffice for its correction. After atropia an J was found sufficient, combined with a cylinder of ~0, to overcome the greater ametropia of the other meridian. It will be found convenient to use the English measures for these calculations, and to reduce Listing’s eye and the other quantities to the following values: Radius, 5 mm. = 0.2 English inch. Anterior focus, 15 “ = 0.6 “ “ Posterior “ 20 “ = 0.8 “ “ The distance apart of apertures in the screen, 4 mm. = 0.16. The distance from observer to test-light, 5 metres = 200 inches nearly, or 16§ feet. The rule, then, would be to multiply the distance from the eye examined to the test-object by the arc of the small angle, and divide the result by the distance apart of the two lights, as ascertained by measurement. Mr. W. suffers from a high degree of myopia, using — J.s, which does not fully correct it. The distance apart of the