Eyesight, good & bad : a treatise on the exercise and preservation of vision.

  • Carter, Robert Brudenell, 1828-1918.
Date:
1880
Catalogue details

Licence: Public Domain Mark

Credit: Eyesight, good & bad : a treatise on the exercise and preservation of vision. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

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    self-correcting, when it is regarded as a source of the sensory impressions which are communicated to the brain. The central part of the retina, at the posterior pole of the eyeball, is called, on account of its colour, the yellow spot, and it is over the yellow spot, that is to say, for the parts of the general image which fall there, that vision is most acute. Within the yellow spot itself there is a central depression in which the acuteness reaches its maximum, and the limit of acuteness appears to de- pend upon the size of the smallest nerve elements of which the retina is here built up. These elements are cone-shaped, and are of extreme minuteness ; and it seems to be essential to the visibility of an object that its retinal image should be large enough to cover at least one cone. The size of the image depends upon two factors, the actual size of the object and its dis- tance ; and these determine what is called the visual angle, or the angle formed between two lines drawn from the extremities of the object to meet at the nodal point or optical centre of the eye, a point near the posterior surface of the crystalline lens. In Fig. 20, C being the nodal point of the eye, and A B an object, A C B is the visual angle of that object, and ab is, the magnitude of its image upon the retina. But the smaller object, a'b', which is nearer to the eye, is seen under the same visual angle and forms an image of the same magnitude ; while the object D E, which is equal in size to AB, but nearer, is seen under the larger visual angle D C E, and forms the larger retinal image de. In