Untitled(_By A. Hilger, Ltd._)The light is brought through one telescope, is split up by the prism,and the resulting spectrum is observed through the other telescope.]But there is a device whereby the power of these giant instruments,great as it is, can be still further heightened. That device is thesimple one of allowing the photographic plate to take the place of thehuman eye. Nowadays an astronomer seldom spends the night with his eyeglued to the great telescope. He puts a photographic plate there. Thephotographic plate has this advantage over the eye, that it builds upimpressions. However long we stare at an object too faint to be seen, weshall never see it. With the photographic plate, however, faintimpressions go on accumulating. As hour after hour passes, the starwhich was too faint to make a perceptible impression on the plate goeson affecting it until finally it makes an impression which can be madevisible. In this way the photographic plate reveals to us phenomena in
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