Elements of natural philosophy (Volume 2-3) . tion of point of the retina affected, and the highest point ofthe above. ±\lQ 0l)ject the lowest of the retina, yet the sensations be-ing referred back along the axes, the points will appearin their true positions and the object to which theybelong erect. In short, instead of the mind contemplat-ing the relative positions of the points in the image, theimage is the- exciting cause that brings the mind to thecontemplation of the points in the of the optic It may be proper to remark here, that the base ofthe optic nerve, where it enters t
Elements of natural philosophy (Volume 2-3) . tion of point of the retina affected, and the highest point ofthe above. ±\lQ 0l)ject the lowest of the retina, yet the sensations be-ing referred back along the axes, the points will appearin their true positions and the object to which theybelong erect. In short, instead of the mind contemplat-ing the relative positions of the points in the image, theimage is the- exciting cause that brings the mind to thecontemplation of the points in the of the optic It may be proper to remark here, that the base ofthe optic nerve, where it enters the eye, is totally insen-sible to the stimulus of light, and the reason assigned forthis is, that at this point the nerve is not yet dividedinto those very minute fibres which are capable of beingaffected by this delicate agent. nerve insensibleto light ELEMENTS OF OPTICS. 237 §72. TJie apparent magnitude of an object is deter- APParent £rjr . magnitude of an mined by the extent of retina covered by its image. object determined;Fig. If, therefore, i? Rr be a section of the retina, by aplane through the optical centre (7, of the eye, andA B—l, ah — X, the linear dimensions of an object andits image in the same plane, we shall have, from thesimilar triangles GAB and C a &, X=- Ga. - s (M)] tr\ Dimension ofimage of anobject on theretina; denoting by e, the distance of the object. And for anyother object whose linear dimension is V and distance s;,calling the corresponding dimension of the image X, I s i Same for a secondobject; and since G a is constant, or very nearly so, X : x •: — : —, s s Proportion; that is, the apparent linear dimensions of objects are astheir real dimensions directly, and distances from the eye inversely. But _-, may be taken as the measure otthe ano-le B C A — bG a, which is called the visual an- 23S NATURAL PHILOSOPHY. Rule second; gle, and hence the apparent linear magnitudes of objectsare said to be directly proportional to their vis
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