Address / by Freeland Fergus, M.D., F.F.P.S.G., F.R.S.E., President, delivered before the Society on 4th November, 1908, at the opening of Session 1908-1909.

  • Fergus, A. Freeland (Andrew Freeland), 1858-1932.
Date:
1908
Catalogue details

Licence: In copyright

Credit: Address / by Freeland Fergus, M.D., F.F.P.S.G., F.R.S.E., President, delivered before the Society on 4th November, 1908, at the opening of Session 1908-1909. Source: Wellcome Collection.

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    distance at which the object is placed. In a limited sense our perception of magnitude depends upon the size of the retinal image, but our consciousness of magnitude is not thus conditioned. In other words, a given size of retinal image does not always give rise to-the sensation of an equal magnitude. Thus, a huge object such as a tower will, at a certain distance from the human eye give the same size of retinal image as a six foot rule, held at a nearer distance. Yet we know that a six foot rule is not equal in size say to the tower of the University, although at different distances the retinal images are of similar dimensions. The sensations got from the size of a retinal image is mentally corrected, and it is mentally corrected very largely although not entirely by the estimation of distance. The apparent size of an object then, is conditioned by the size of the retinal image, and conditioned by the distance of the object from the person who sees it. But Berkeley misses, all but entirely, the most important function of vision, namely, that property to which we give the name of visual acuteness. Without visual acuteness, as already stated, man would be unable to read or to write, and would be much curtailed in his energies. The only reference which is made to something like visual acuteness occurs in the 83rd paragraph. “The visive faculty, considered with reference to its immediate objects, may be found to labour of two defects : first, in respect of the extent or number of visible points that are at once per¬ ceivable by it, which is narrow and limited to a certain degree. It can take in at view but a certain determinate number of minima visibilia, beyond which it cannot extend its prospect. Secondly, our sight is defective in that its view is not only narrow, but also for the most part confused; of those things that we take in at one prospect, we can see but a few at once clearly and unconfusedly and the more we fix our sight on any one object, by so much the darker and more indistinct shall the rest appear.” Almost within our own time Professor Snellen of Utrecht tried to define the smallest object that could be seen, and on his researches founded those test-types which are all but universally used as a measure of visual acuteness. Unfortunately they have come to be regarded as an absolute standard instead of being, as they are, only an extremely useful comparative one, and the normal visual acuteness of mankind has not yet been determined.