American journal of ophthalmology. . ly these principles tothe instrument as now made we find that they do not hold inpractice, for we have the crossing of the bands in the meridianof greatest curvature where the corneal image is the instance, the bands being in contact in the horizontalmeridian, if the arc is turned to the vertical meridian and thereis a crossing of two steps, that signifies that we have an astig-matism of 2D, according to the rule, in which the verticalmeridian of the cornea is the most strongly curved and mosthighly refracting. This seems in direct opposition t
American journal of ophthalmology. . ly these principles tothe instrument as now made we find that they do not hold inpractice, for we have the crossing of the bands in the meridianof greatest curvature where the corneal image is the instance, the bands being in contact in the horizontalmeridian, if the arc is turned to the vertical meridian and thereis a crossing of two steps, that signifies that we have an astig-matism of 2D, according to the rule, in which the verticalmeridian of the cornea is the most strongly curved and mosthighly refracting. This seems in direct opposition to the properoptical theory, for in accordance with this, the image beingsmaller they should separate. This they do, but by the new 2For calling my attention to it I have to thank one of my former pupils, Prof. Shute, , of this cily. 300 .Journal of Ophthalmology. arrangement in the later instruments as the image growssmaller the adjacent bands encroach one upon the other as isreadily understood from the accompanying Fig. i. In A we have the object a b, the band a, which representsone end being rectangular, the other end b having gradationsin steps. This is as it appears, one the arc of the ophthalmo-meter as seen by the examine. When the cornea is viewedby the observer through the telescope and prism,—the arcbeing in a horizontal position — the two images of the cornealreflection are seen inverted in the relative positions as shownin B, where a b is one image and a, b is the other. Theinner side of a is in contact with the inner side of b. Thedistance between the inner sides of each of these images is 3mm. when the instrument is properly adjusted. Now let thearc be turned to the vertical meridian which is the more Original Articles 301 strongly curved and more highly refracting. According tothe optical law, the images a b and a b should be smaller,and a should approach b and a should approach b, and indoing this of course a and b will overlap as shown in C
Size: 1698px × 1471px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectophthalmology, bookye
