Page:Popular Astronomy - Airy - 1881.djvu/58

44 would see the object. Instead of looking at the screen from a distance, you may come close to it and view it with an eye-glass on the side opposite to to the object glass, and then the resemblance to the telescope is complete. You see upon it bars representing wires in the field of view. The object of these is to give definiteness and distinctness to the observations. Supposing I use the telescope by directing it to a star: if I see a star somewhere in the telescope, this is a very loose observation, because I have not sufficiently defined the place; but if I have wires in the telescope, and observe the star on any one of these wires, then I have observed it in a definite part of the field of view. The accuracy gained by that observation is very great indeed; it is the most important adjunct connected with the use of the telescope. Every surveyor knows the value of the wires in his theodolite telescope.

There was another thing I pointed out, which was that the rotation of the stars, when it is examined closely, is not so accurate as might be supposed at first, for this reason: that we always find that the stars near the horizon appear higher than they really are in fact—whether east, north, or west, they are always a little too high—I ascribed that to refraction. And I pointed out, as a law of refraction, by reference to a glass trough or prism of water, that if the light falls on a surface of glass, it is bent there in such a manner as to go more square to the surface. I had occasion in the last lecture to mention strongly my disapproval of the use of some words in a wrong sense. I shall now mention another word which is often used in a wrong sense: I allude to the word "perpendicular." Many people think that the word "perpendicular" means the position of a plumb-line.