Page:Astronomy for Everybody.djvu/75

at a bright star through any large refracting telescope, you will see it surrounded by a blue or purple radiance. This is produced by the blue or violet light which the two lenses will not bring to one focus.

By the action of the objective, in thus bringing rays to a focus, the image of a distant object is formed in the focal plane. This is a plane passing through the focus at right angles to the axis or line of sight of the telescope.

What is meant by the image formed by a telescope can be seen by looking into the ground glass of a camera with the photographer, as he sets his instrument for a picture. You there see a face or a distant landscape pictured on the ground glass. To all intents and purposes the camera is a small telescope, and the ground glass, or the point where the sensitive plate is to be fixed to take a picture, is the focal plane. We may state the matter in the reverse direction by saying that the telescope is a large camera of long focus, with which we can take photographs of the heavens as the photographer takes ordinary pictures with the camera.

Sometimes we can better comprehend what an object is by understanding what it is not. In the celebrated moon hoax of half a century ago or more, there was a statement which illustrates what an image is not. The writer said that Sir John Herschel and his friend finding that, when they used enormous magnifying power, there was not light enough for the image to be visible, the