INTRODUCTION TO OPTICS. Ixxvii 



bodies send rays of Jig-lit immediately to our eyes ; but the rays which 

 they send to other bodies are invisible to us, and are seen only when 

 reflected or transmitted by those bodies to our eyes. Yet it may be 

 observed that the ray of light on its passage from the sun to the mirror, 

 arid its reflection, have been spoken of as visible, though in neither case 

 were those rays in a direction to enter our eyes. The fact is, that what is 

 seen is the light reflected to the eye by small particles of dust floating in 

 the air, and on which the ray shone in its passage to and from the 

 mirror. So when, in common phrase, we speak of seeing the sun shining 

 on an opposite house, it is impossible to see a single ray which passes 

 from the sun to the house : no rays are visible but those which enter our 

 eyes ; therefore it is the rays which are reflected by the house, and not 

 those which proceed from the sun to the house, that are seen. Why, 

 then, does one side of the house appear to be in sunshine, and the 

 other in the shade ; for if we cannot see the sun shine upon it, the 

 whole of the house should appear in the shade ? That side of the house 

 on which the sun shines reflects more vivid and luminous rays than the 

 side which is in shadow, the latter being illumined only by rays reflected 

 upon it by other objects : these rays are therefore twice reflected before 

 they reach our sight ; and as light is more or less absorbed by the bodies 

 it strikes upon, every time a ray is reflected its intensity is diminished. 

 Thus, on a large sheet of water the sun appears to shine on one part 

 only, though the whole of it is equally exposed to its rays. This partial 

 brilliancy of water is more remarkable by moonlight, on account of the 

 deep obscurity of the surrounding parts. To account for this, it must be 

 remembered that the direction of a reflected ray depends on that of the 

 incident ray; the sun's rays, therefore, which fall with various degrees of 

 obliquity upon the water, are reflected in directions equally various : some 

 of these will meet the eye, and it will see them, but those which fall 

 elsewhere are invisible to it*. 



Let us now examine by what means the rays of light produce vision. 

 They enter at the pupil of the eye, and proceeding 1 to the retina or optic 

 nerve, which is situated at the back of the eye-ball, there describe the 

 figure, colour, and (with the exception of size) form a complete repre- 

 sentation of the object from which they proceed. If the shutters be 

 closed, and a ray of light admitted through the small aperture, a picture 

 may be seen on the opposite wall similar to that which is delineated on 

 the retina of the eye: it exhibits a picture in miniature of the garden, and 

 the landscape would be perfect were it not reversed. This picture is 

 produced by the rays of light reflected from the various objects in the 

 garden, and which are admitted through the hole in the window-shutter* 

 It is called a camera obscura {dark chamber}, from the necessity of 

 darkening the room in order to exhibit it. 



The rays from the glittering weathercock at the top of the alcove, A 

 (fig. 7), represent it at a; for the weathercock being much higher than 

 the aperture in the shutter, only a few of the rays, which are reflected by 

 it in an obliquely descending direction, can find entrance there. The 

 rays of light moving always in straight lines, those which enter the room 

 in a descending direction will continue their course in the same direction, 

 and will, consequently, fall upon the lower part of the wall opposite the 

 aperture, and represent the weathercock reversed in that spot, instead of 

 erect in the uppermost part of the landscape ; and the rays of light from 

 the steps, B, of the alcove, in entering thejaperture, ascend, and describe 

 * See Mechanics. 



