Page:Astronomy for Everybody.djvu/71

There is no branch of science more interesting to the public than that with which the telescope is concerned. I assume that the reader wishes to have an intelligent idea as to what a telescope is and what can be seen with it. In its most complete form, as used by the astronomer in his observatory, the instrument is quite complex. But there are a few main points about it which can be mastered in a general way by a little close attention. After mastering these points, the visitor to an observatory will examine the instrument with much more satisfaction than he can when he knows nothing about it.

The one great function of a telescope, as we all know, is to make distant objects look nearer to us; to see an object miles away as if it were, perhaps, only as many yards. The optical appliances by which this is effected are extremely simple. They are made with large well-polished lenses, of the same kind as those used in a pair of spectacles, differing from the latter only in their size and general perfection. A telescope requires an appliance for collecting the light coming from the object so as to form an image of the latter. There are two ways in which the light may be collected, one by passing the light through a set of lenses, and one by reflecting it from a concave mirror. Thus we have two different kinds