Cases of congenital amblyopia with visual acuteness of 1/7 are so frequent, that I have not drawn up special statistics of them. I was not anxious to collect a large number of cases but only material for evidence. I have therefore divided the 98 cases I observed into 3 groups. (1) Cases with visual acuteness of less than 1/7 to V. 1/12; (2) V. less than 1/12 to V. 1/36; (3) visual acuteness less than 1/36. The limits between these groups are of course not very sharply defined, for what is designated as "measurement" of visual acuteness contains, even if we accept the statements of patients as trustworthy, not an inconsiderable number of sources of error; and we often find a remarkable absence of conformity in the analysed causes of congenital amblyopia, according as we seek to determine the visual acuteness by means of single test-letters or by reading printed matter. In a case of visual acuteness of 1/12 No. 0·75 with convex 6 was the smallest type that could be read, and that with difficulty, larger type was usually required; and in one case where at first only single words of No. 2·25 were read with difficulty—this test was on that account repeated in myosis by eserine—No. 1·75 was finally the smallest print which could with the same difficulty be deciphered. In the division of the groups here arranged the best visual acuteness ascertained in the various examinations was always used as the basis.
A. Vision less than 1/7 to V. 1/12 38 cases. The examination of the better eye showed:
(a) Emmetropia in 18 cases. A determination of refraction, mostly ophthalmoscopic, of the weaker eye is submitted in 11 cases, which divide themselves into, 4 with emmetropia, 3 with hypermetropia (of H. 2 D. and 2·25 D.), 3 with hypermetropic astigmatism, I with myopic astigmatism.
(b) Myopia in 5 cases (3 of M. 1 D. to 1·5 D., 2 of M. 4·5 D. and 4 D.), the condition of the defective eye was determined in 3 cases, and was twice hypermetropic, once astigmatic.
(c) Hypermetropia in 8 cases, hypermetropic astigmatism in 3. In 4 cases an exact determination of refraction even of the better eye was for some reason impracticable.
There are 4 cases in this group where the visual acuteness in both eyes did not exceed the above-stated small amount, and one which was interesting from another point of view.
Case 19.—Max L—, æt. 8-1/2, recognises No. 24, and a few letters of 18 at 5 metres with the better eye with convex 6 D.; at 1 metre V. 1/4 to 1/3, the left eye recognises only No. 60 at 5 m. with + 6 D. at 1 m. No. 0·75 is read with difficulty. If we exclude one eye it lapses into now less, now greater convergence, and still no squint is present, but diplopia as well as binocular fusion can be proved by the aid of prisms. The theory of Donders that squint is less frequent in hypermetropia of high degree because too strong convergence would not suffice to furnish clear retinal images, is scarcely tenable in the face of such cases. If indistinct retinal images are added to a visual acuteness of only 1/3 to 1/4 still, even with faulty accommodation, it is difficult to believe how a child could learn to read if it did not hold the book close to its eyes, which was not the case here, and indeed seldom happens. Therefore, in spite of defective vision the accommodation must have sufficed, without sacrificing binocular fusion, whilst in all probability accommodative convergence followed on exclusion of one eye.
B. 48 cases had visual acuteness from 1/12 to 1/36. The better eye was—
(a) Emmetropic in 16 cases; in 6 of them the refraction of the defective eye was determined, which showed in one case emmetropia, 3 hypermetropia, 2 astigmatism.
(b) Myopia of the better eye was present in 7 cases (in 3 myopia of 1 D., in 4 M. 3 D. to 6 D.).