Extra-papillary colobomata / by G. Lindsay Johnson.

  • Johnson, G. Lindsay (George Lindsay), 1853-1943.
Date:
[1890]
Catalogue details

Licence: Public Domain Mark

Credit: Extra-papillary colobomata / by G. Lindsay Johnson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    mark that, in all those cases in which the vision was normal and in some of the cases where it was deficient, I have found the difference of the focus between the margin of the coloboma and the floor amounted to between 2 D and 3 D. By Dr. Landolt's tables for axial ametropia, based on the well-known formula \/ — cp =-^-^ we find, by placing a lens of — I D at the anterior principal focus of an emme- tropic eye, that the image is removed to a point 0.321 mm behind the retina. From the table of eye measurements in Quain's Anatomy we find the thickness of the sclera, choroid, and retina together = 0.08 in., and of the sclerotic alone = 0.05 in., giving for choroid and retina together a thickness of 0.03 in. = 0.75 mm. Now, if 0.321 jnm corre- sponds to a correcting lens of — i D, a plane removed 0.75 mm farther back will need a concave lens of 2.5 D placed at the anterior focus.' This closely coincides with the actual lens used in the ophthalmoscope to bring the floor of the coloboma into focus. Of course, these figures are only to be considered approximately true, as we find it impossible to focus the fundus accurately within half a ^ dioptry, and, moreover, it is open to doubt whether the thickness of the retina ought to be taken into account at all; but, making every allowance, I think it shows that in many cases of coloboma the depression was not an ectasia or staphyloma, but was due to the absence of retina and choroid, enabling us to see the sclerotic in its unaltered position. Vision.—If the reader takes the trouble to glance over the notes of the cases appended he cannot but be struck with the excellence of the vision in many of the cases. Thus, in the case of Sarah S (Figs. 7 and 8, Plate IV.) and of Ralph T (Fig. 10, Plate V.) the vision was above normal = |- in both eyes, while in that of George P (Fig. i) and George G (Fig. 3), as well as in Professor Tweedy's two cases (see diagrams), the vision — -|. In Arthur Y 's case the vision was nearly |-. This was ^ The anterior principal focus is situated 13.7 mm in front of the cornea, which is as nearly as possible the position of the correcting lens of the oph- thalmoscope when used in the direct examination of the fundus. %