Then suppose we put this glass in the tube of an opera-glass, or pocket spy-glass, and look through the eye-hole and the concave lens, properly adjusted, in front of it, we shall see the image of the object considerably magnified.

But suppose the object draws very near, we see nothing distinctly; for the rays reflected from it, which were nearly parallel while it was at a distance, are no longer so when it comes near, but scatter in all directions, and those which fall on the lens are collected at a point much nearer to the lens than before, and the eye-glass must be pushed forward to that focus.

Accordingly, you know that the spy-glass is made to slide back and forward, and the telescope has a screw to lengthen or shorten the tube according to the distance of the objects observed.

Another way of meeting the case would be by taking out the lens, and putting in one of less magnifying power, a flatter lens, for the nearer object.

Now, at first sight, it would seem a very inconvenient thing to have eyes drawing out and in several inches like spy-glasses, and still more inconvenient to have twenty or thirty pairs of eyes, and to need to take out our eyes, and put in a new set twenty times a day.