Page:Astronomy for Everybody.djvu/76

friend suggested that the image should be illuminated by artificial light. This was done with such brilliant success that animals in the moon were made visible through the telescope. If many people, even those of the greatest intelligence, had not been deceived by this, I should hardly deem it necessary to say that the image of an object formed by a telescope is such that, in the very nature of things, extraneous light cannot aid in its formation. Its effectiveness does not proceed from its being a real image, but only from the fact that all the rays from any one point of a distant object meet in a corresponding point of the image, and there diverge again, just as if a picture of the object were placed in the focal plane. The fact is that the term picture is perhaps a little better one than image to apply to this representation of the object, only the picture is formed by light and nothing else.

If an image or picture of the object is thus formed so as to stand out before our eyes, one may ask why an eyepiece is necessary to view it; why the observer cannot stand behind the picture, look toward the objective and see the picture hanging in the air, as it were. He can really do so if he holds a ground glass in the focal plane, as the photographer does with the camera. He can thus see the image formed on the glass. If he looks into the object-glass he can see it without any eyepiece. But only a very small portion of it will be visible at any one point, and the advantage over looking directly at the object will be slight. To see it to advantage an eyepiece must be used. This is nothing more than a little eye