File:Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct - for the use of schools and colleges (1870) (14780319144).jpg

Identifier: outlinesofcompar00agas (find matches)
Title: Outlines of comparative physiology touching the structure and development of the races of animals, living and extinct : for the use of schools and colleges
Year: 1870 (1870s)
Authors: Agassiz, Louis, 1807-1873 Gould, Augustus A. (Augustus Addison), 1805-1866 Wright, Thomas, 1809-1884
Subjects: Physiology, Comparative Zoology
Publisher: London : Bell & Daldy
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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h is to the rightappears to the left in the image.*As every object emits rays fromevery point in all directions, whichthen proceed in straight lines, theaxal rays e, f, g, of the differentpencils proceeding from either end,and the middle of the arrow, a, b,must cross at some point withinthe eye. Numerous observationssatisfy us that this point lies verynear the centre of the eye (h),somewhat behind the crystallinelens (#). The prime rays, e, f, g, which proceed fromthe object may be^named, in reference to the eye, rays ofdirection, because every prime or axal ray of a pencil de-termines the direction of the* other rays, in order that all ofthem may meet in a focus upon the retina. The point at * (It is most easy to obtain conviction of this reversed position of objectsupon the retina, by taking the eye of a white rabbit, free from pigment,clearing the globe from fat, muscles, &c., and then presenting it with thecornea in front to the window; all the objects before it, such as trees
Text Appearing After Image:
62 SPECIAL SENSES. Fig. 39, which the rays must diverge, if a clearly defined image is tobe formed, is called the point of intersection, or focal centre.*The position of this point is determinable, with the assistance ofan instrument for measuring angles ; it lies somewhat behindthe crystalline lens, and very near the centre of the eye. Theintersecting axal rays of two objective points (fig. 38) inclosean angle (a, h, b, for the object a, b; i, k, h, for the objecti, k), which is called the visual angle. This angle diminisheswith the distance of the two objects from the eye, and theretinal image is in the same proportion smaller. The arrow,i, k, is only half the distance of the arrow, a, b, from the eye;the visual angle, i, h, k, is therefore twice as large as the anglea, h, b, and the same thing is true in reference to the imagesdepicted upon the retina. It is on this account that objects ofdifferent magnitudes seen at different distances, but of which thevisual angles are the same,

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