42 



OPTICS. 



From the dimensions of the eye given the progress of rays through the hu- 

 above, and by means of the preceding mours of the eye, whether they fall upon 

 indices of refraction, it will be easy to it in a parallel or a diverging condition. 



trace, by the method already described, 



Fig. 43 



diverging condition. 



IN, for example, 



be an object at a considerable distance 

 from the eye, E F O. Rays of light 

 diverging from the points M N, will be 

 converged by the refraction of the hu- 

 mours to points ?n, n upon the retina, 

 where they will form an inverted image 

 of it, in the same manner as an image 

 is formed in a camera obscura. That 

 such an image is actually formed on the 

 back of the eye, may be easily proved 

 by paring away the sclerotic coat of the 

 eye of an ox, with a sharp knife, till it 

 is sufficiently thin to allow the image to 

 be seen through it, 



In what manner the retina, thus im- 

 pressed with a distinct image of an ex- 

 ternal object, conveys to the mind, 

 through the medium of the optic nerve, 

 of which it is the expanded termination, 

 a knowledge of the existence, the po- 

 sition, and the magnitude of that object, 

 is not known, and probably never will 

 be. Certain facts, however, or laws 

 of vision, have been deduced from ob- 

 servation, and merit our attentive con- 

 sideration. 



1 . On the direction of visible objects. 

 When the mind sees the extremity M 

 of any object M N, by means of rays 

 flowing from M and collected at m, the 

 retina receives these rays at different 

 degrees of obliquity, and yet the point 

 3VI is seen only in one direction, namely, 

 in the direction of the central ray of the 

 cone whose apex is at m. This however 

 does not arise from the ray being the 

 resultant, as it were, or the mean of the 

 directions of all the other rays ; for if we 

 close up all the pupil excepting a small 

 opening at its margin, the point M will 

 be represented at m only by the most 

 oblique rays of the conical pencil, and 

 yet it will still be seen in the same di- 

 rectio nas before. Hence we conclude, 

 that when a ray of light falls upon any 

 point m of the retina, in any direction, 

 however oblique to its surface, the ob- 

 ject will be seen in the direction of a 

 line perpendicular to the retina at the 



point m. As the surface of the retina 

 is a portion of a sphere, these perpen- 

 diculars must all pass through one 

 point, which may be called the centre of 

 visible direction ; because every point of 

 an external object will be seen in the 

 direction of a line joining that centre 

 and the given point. The truth of this 

 we have established by marking the 

 perfect stability of the image of any ob- 

 ject, when it is seen by different points of 

 the retina when the eyeball alone is 

 moved. Hence the centre of visible 

 direction is a fixed point in the vitreous 

 humour ; and, as it never changes its 

 place during the rotation of the eyeball, 

 it must be coincident with the centre 

 round which that rotation is performed. 

 In consequence of this coincidence, and 

 in virtue of the law of visible direction, 

 an arrangement of consummate skill, 

 the great Author of nature has provided 

 for the perfect stability of every point in 

 the images of external objects. 



2. Cause of erect vision. As the 

 humours of the eye act exactly like a 

 convex lens of an equivalent focal 

 length, an inverted picture of external 

 objects will, for the reasons already 

 assigned (Chap. iv. p. 12), be formed 

 upon the retina. Many philosophers 

 of eminence have perplexed themselves 

 very unnecessarily, in attempting to de- 

 duce erect vision from inverted images. 

 The law of visible direction removes at 

 once every difficulty ; for as the lines of 

 visible direction must necessarily cross 

 each other at the centre of visible di- 

 rection, those from the lower part of the 

 image must go to the upper part of the 

 object, and those from the upper part 

 of the image go to the lower part of the 

 object, and hence an erect object is the 

 necessary result of an inverted image. 



3. Distinct and indistinct visio?i in 

 the same object. When we look in- 

 tensely at any point of an object in or- 

 der to examine it with care and atten- 

 tion, we direct to that point the axis of 



