An elementary treatise on the function of vision and its anomalies / by Dr. Giraud-Teulon ; translated from the second French edition by Lloyd Owen.

  • Owen Lloyd.
Date:
1880
Catalogue details

Licence: Public Domain Mark

Credit: An elementary treatise on the function of vision and its anomalies / by Dr. Giraud-Teulon ; translated from the second French edition by Lloyd Owen. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    any definite direction. Now it can only do this by virtue of that faculty which the eye possesses of seeing the entire picture at a single glance, and without which it could not seize the relative situation of objects. The faculty of orientation (the judgment of the relative position of objects) is, therefore, derived pre- cisely from the property which the retina has of grasping instantaneously the whole extent of the space.* § II. Tlie Focms oftflie Eye wlien fliat org-aii is in a state of repose.—Man and a large number of the superior animals have the faculty of seeing clearly on the horizon. In this condition distinct vision takes place without effort and as the effect of the power of refraction alone, corresponding absolutely with the structure of the transparent media of the organ (static or passive refraction). The rays which strike the cornea are then parallel, and the eye repre- sents exactly the camera obscura at the moment when * An eye reduced to central vision is in the greatest perplexity in guiding itself; it is in the condition of an astronomer whose telescope is deprived of the additional instrument called the finder. In order to discover a star in the expanse of the celestial sphere, it must be possible to embrace in a single glance all .the region in which it is known to be found, a thing almost impossible with the telescope, in which the superficial field is extremely limited. It is accomplished readily by means of the finder, which possesses a much more extended field of vision, and whose axis is parallel to that of the telescope to which it is attached.