1890.] on Astronomical Telescopes. 159 



is greater, and the angular dimension is converted into a linear one 

 on the screen in due proportion. Now, as we shall assume that the 

 eye sees all things best at the distance of about nine inches, we may 

 say that the picture taken with a lens of this focal length gives at 

 once the proper and most natural representation we can possibly 

 have of anything at which we can look. Such a picture of a land- 

 scape, if placed before the eye at the distance of nine inches, would 

 exactly cover the real landscape point for point all over. A picture 

 taken with a lens of shorter focal length, say four inches, will give a 

 picture as true in all the details as the larger one, but if this picture 

 is looked at, at nine inches distance, it is not a true representation of 

 what we see ; in order to make it so, we must look at it with a 

 lens or magnifier. With a larger picture one can look at this at the 

 proper distance, which always is the focal distance cf the lens with 

 which it was obtained, when we will see everything in the natural 

 angular position that we have in the first case. 



But if, instead of looking at this larger picture, which we may 

 consider taken with a lens of say ninety inches focal length, at a dis- 

 tance of ninety inches, we look at it at a distance of nine inches, we 

 have practically destroyed it as a jjicture by reducing the distance at 

 which we are viewing it, and we have converted it into what is for 

 that particular landscape a telescopic picture ; we see it, not from the 

 point at which it was taken, but just as if we were at one-tenth of 

 the distance from the particular part that we examine. A telescope 

 with a magnifying power of ten, would enable us to see the landscape 

 just as we see it in the photograph, when we examine it in the way I 

 have mentioned. 



Having thus seen how a lens or mirror acts, we will turn our at- 

 tention to the eye. Here we find an optical combination of lenses 

 that act together in the same way as the single convex lens of which 

 we have been speaking. We will call this combination the lens of 

 the eye. It produces a picture of distant objects which in the normal 

 eye falls exactly in focus upon the retina. We are conscious that 

 we do see clearly at all distances beyond about nine inches. 



At less than this distance objects becomes more and more indistinct 

 as they are brought nearer to the eye. From what we have seen of 

 the action of the lens in producing pictures of near and distant objects, 

 we know that some movement of the screen must be made in order to 

 get such pictures sharply focussed, a state of things necessary to perfect 

 vision. We might therefore suppose that the eye did so operate by 

 increasing when necessary the distance between the lens and retina, 

 but we know that the same effect is produced in another way ; in fact, 

 the only other way. The eye by a marvellous provision of nature, 

 secures the distinctness of the picture on the retina of all objects 

 beyond a distance of about 9 inches, by slightly but sufficiently varying 

 the curvature of one of the lenses ; by an effort of will, we can make 

 the accommodating power of the eye slightly greater, and so see things 

 clearly a little nearer ; but at about the distance of 9 inches, the 



