Ixxviii 



INTRODUCTION TO OPTICS, 



them in the highest instead of the lowest part of the landscape ; whilst 

 the rays proceeding from the alcove, which is to the left, describe it on 

 the wall to the right. Those which are reflected by the walnut-tree, C D, 

 to the right, delineate its figure in the picture to the left, c d. Thus the 

 rays, coming in different directions, and proceeding always in straight 

 lines, cross each other at their entrance through the apertures : those 

 from above proceed below, those from the right go to the left, those from 

 the left towards the right; thus every object is represented in the picture 

 as occupying a situation the very reverse of that which it does in nature, 

 excepting the flower-pot, E F, which, though its position is reversed, 

 does not change its situation in the landscape ; for being immediately in 

 front of the aperture, its rays fall perpendicularly upon it, and conse- 

 quently proceed perpendicularly to the wall, where they delineate the 

 object. It is thus that the picture of objects is painted on the retina of 

 the eye. The pupil of the eye, through which the rays of light enter, 

 represents the aperture in the window-shutter ; and the image delineated 

 on the retina is exactly similar to the picture on the wall. 



The retina of the eye exhibits a much more perfect image than any 

 mirror : the extensive landscape beheld from the window is there repre- 

 sented with the greatest accuracy. Art would in vain attempt to paint 

 so small and distinct a miniature ; but Nature works with a surer hand 

 and a more delicate pencil. That Power which forms the feathers of the 

 butterfly and the flowerets of the daisy can alone pourtray so admir- 

 able and perfect a miniature. As the rays intersect each other on entering 

 the pupil, in the same manner as they do on entering the camera obscura, 

 the image is reversed. The scene, however, does not excite the idea 

 of being inverted, because we always see an object in the direction of the 

 rays which it sends to us. How it is that we do so is a point rather diffi- 

 cult to explain clearly. The following, however, seems to be the best 

 explanation : A ray which comes from the upper part of an objet de- 

 scribes the image on the lower part of the retina; but experience having 

 taught us that a ray which strikes the retina there comes from above, we 

 consider that part of the object it represents as uppermost. The rays 



