Clinical Investigations on Squint. C. Schweigger
According to this the percentage of the hypermetropia (including doubtful cases) amounts to 66 per cent. Dr. Isler in his dissertation, 'The Dependence of Strabismus on Refraction,' gives the percentage of hypermetropia in convergent squint as 88 per cent.—a great difference, which can, however, be partly accounted for. Isler found in hypermetropia of 2 to 10 dioptres squinting in 75 per cent.; in my statistics H. 1·5 D. to the highest degrees of hypermetropia are likewise represented by 75 per cent. As the difference between H. 2 D. and H. 1·5 D. amounts to only half a dioptre, the results of the statistics agree perfectly within these limits; the difference lies only in the slighter degrees of hypermetropia, for the diagnosis of which refer to pp. 12 to 14.
The influence of hypermetropia is very apparent in the percentage of periodic squint. While in myopia, emmetropia, and slight hypermetropia, the sum total of permanent as compared to periodic squint is as 100: 19·5, this number mounts in hypermetropia of 1 D. to H. = 3 D. to 52·5 and in the higher degrees to 59 per cent. Despite the small number of cases it is probably no mere accident that in the highest degrees (of H. = 5 D. and more) this percentage is calculated at 77·7.
But just this undoubted favouring of periodic squint by hypermetropia, helps to show that this condition is one of the causes of squint, but not the only one, for in periodic squint just those conditions are wanting which induce a permanent deviation.
It is further proved by the table that in convergent strabismus, myopia appears just about as frequently as the higher degrees of hypermetropia (of 3 dioptres and more). The fact that these are not so strongly represented in convergent strabismus, as one would have expected according to his theory, had also struck Donders. "This cannot be wondered at," he continues, "the power of accommodation, even with increased convergence, does not here suffice to produce clear images. One gains much better