Visual optics and sight testing / by Lionel Laurance.
- Laurance, Lionel, 1856?-1936.
- Date:
- 1912
Licence: In copyright
Credit: Visual optics and sight testing / by Lionel Laurance. 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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![reverse. If there be diplopia, and a red glass be placed in front of the one eye, a candle flame seen by it usually appears nearer than the other. Never- theless, when the distances of differently coloured bodies are compared, what appears nearest to one person may not seem so to another. Convergence and accommodation also unconsciously help in judgment of size. If an unknown object causes a certain-sized retinal image, and requires a certain effort of convergence and accommodation in order to be clearly seen, it is conceived to be of certain dimensions. The same retinal image, combined with smaller muscular efforts, would give the appearance of greater size, while greater efforts would cause it to be thought smaller. If, however, in order to cause accommodative effort a concave lens be placed before a normal eye, the object generally appears more distant, notwithstanding the extra muscular effort. In this case the diminution in the size of the retinal image more than counteracts the sense of nearness produced by the accom- modation. For a similar reason a convex lens may cause the sensation of nearness, since the size of the retinal image predominates and the effect of increased distance, due to the suppression of accommodation, is lost. The effect of a telescope or opera glass is to increase the size of the visual angle without calling into play any muscular effort. An actor on the stage seen through an opera glass simply appears to be nearer by an amount equal to the magnification of the glass. If the latter enlarges three diameters, it apparently brings objects up to a third their actual distance, and so on. Looking at an object through an opera glass the reverse way causes the impression of increase of distance equal to the magnifying power. The conceived or apparent size of an object depends also on the distance to which its image is mentally projected, and generally the plane of the image is the same as that of the object. If, however, the plane of the ob]ect be unknown, this may, or may not, result. If possible, the mind always uncon- sciously seeks a plane on which to project the image. Thus the figures in a picture are projected on to the picture, the image of the whole picture itself is projected on to the wall on which it hangs. The mental image of a real aerial image, formed by a lens or mirror, is projected on to the plane of the lens or mirror, as being the nearest tangible body. The distance and there- fore, the size of a distant small body lying in front of a large one is difficult to estimate, as the image of the small body is projected on to the larger on. In other words, we cannot appreciate the gap between two distant bodies situated in the line of sight. Comparative Size.-The estimation of the comparative sizes of various objects depends on their images being formed successively on the macuk by means of rapidly achieved movements of the eyeball, so that the dflerent retinal areas stimulated by the images are mentally compared. If.ob^s^e close together and in the same plane, comparison of their size*, «.easy, but not so if they are widely separated or in different plane, ^ouf Jh myope has a large retinal image and the hypermetrope a small one, the com](https://iiif.wellcomecollection.org/image/b21287582_0060.jp2/full/800%2C/0/default.jpg)
