| Visual acuteness | to 1/7 | 121 | ||
| " | less than | 1/7 to V. 1/12 | 17 | |
| " | " | 1/12 to V. 1/36 | 27 | |
| " | " | 1/36 | 12 |
Defective sight then, 31·6 per cent.
(c) In hypermetropia 3 D. and more, 70 cases with convergent squint, with:
| Vision to 1/7 | 39 |
| V. < 1/7 to V. 1/12 | 8 |
| V. < 1/12 to V. 1/36 | 14 |
| V. < 1/36 | 9 |
Defective sight then, 44·2 per cent.
This regular increase of defective sight with the increase of the hypermetropia can be no mere accident, and speaks strongly for the identity of defective vision in squint with congenital amblyopia. Were defective vision caused by the squint the various states of refraction would show no difference in the percentage of defective vision.
Further, the circumstance is worthy of remark that among 198 cases of periodic squint (convergent and divergent) which are applicable for the statistics of visual acuteness—
| 170 | possess | V. to 1/7. |
| 16 | " | V. < 1/7 to V. 1/12. |
| 9 | " | V. < 1/12 to V. 1/36. |
| 3 | " | V. < 1/36. |
| 14·2 per cent. then of defective vision of considerable degree. | ||
That defective sight on the whole plays an influential part as a cause of squint is doubted by no one, indeed we see blind eyes lapse into squint as soon as the conditions necessary to it are supplied by the muscles. Of all the prevailing causes present defective vision will be the more decisive in proportion as it is of high degree; for the motive which despite the presence of favouring circumstances can prevent the real occurrence of squint, binocular vision, becomes less efficacious as the defective vision becomes more considerable. As binocular fusion takes place frequently in periodic squint, for a time at least, that is as long as proper fixation lasts, one can understand that periodic squint exists chiefly in cases where the visual faculty of both eyes is good. Even the highest degrees of congenital amblyopia are not excluded, for periodic squint appears where the faculty of binocular fusion has been completely lost. Further, that considerable congenital defective sight is more frequent with than without squint, may be accounted for quite simply by the fact that, in extreme degrees of it, binocular fusion cannot be learnt at all, while in the lesser degrees it is more easily forgotten again.
If defective vision is undoubtedly one of the causes of squint, we must seek for the grounds upon which it has been taken to be a consequence of squint, and described as amblyopia ex anopsia. I will not inquire to whom the honour of this invention belongs. I do not want to write a history of mistakes but only to examine the basis of the views now current. The most complete record of the same may be found in the well-known journal on the 'Cure of Eye Diseases,' vol. v, p. 1011. Leber, who does not seem to recognise the existence of congenital amblyopia, has shown quite a special predilection for amblyopia ex anopsia.