THE OPTICAL DEFECTS OF THE EYE. 



11 



Before leaving the consideration of optical lenses, there is one 

 subject to which I wish to direct your attention ; namely, the for- 

 mation of an inverted image behind a convex lens. 



Many of you are, probably, familiar with the fact, that when light 

 is admitted into a darkened room, through a small orifice, there ap- 

 pears upon the opposite wall of the room, an inverted, dim, shadowy 

 picture of buildings, trees, &c., in front of the aperture. This can 

 also be seen, on a smaller scale, by holding a sheet of white paper a 

 few inches from the key-hole of a darkened hall. 



The philosophy of this is seen in Eig. 5. 



Let A, B, represent the position of a flame of a lamp that is a 

 short distance in front of an aperture of a darkened box. Pencils 

 of divergent rays of light radiate from the apex of the flame in every 

 direction ; one of these pencils is represented in the figure to illu- 

 minate the end of the box, and one of the rays escaping through the 

 small orifice c ; this ray passes in a straight line to the back of the 

 box, and strikes the point a, which it illuminates. 



Says of light diverge from the lower part of the flame, also ; one 

 of these rays is shown to enter the aperture c, and to pass to the 

 back of the box a't h. In a similar way it might be illustrated that 

 pencils of light radiate from every point in the flame A, B, and that 

 one ray from each point passes into the box and illuminates a por- 

 tion of the back. In this way we get an illuminated spot at the 

 back of the box, which is an exact counterpart of the flame in front 

 of the box, but inverted, the apex of the flame pointing downwards. 

 The reason that the picture is reversed is that, as rays of light (in 

 the same medium) pass in straight lines, ' a ray from the top of the 

 flame, after passing the aperture, must necessarily pass to the lower 

 part of the back of the box ; and a ray from the lower part of the 

 flame must necessarily (in moving in a straight line) pass to the 

 upper part of the back of the box. You will observe, also, that the 



