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of the magnifying power. For example, if we have a magnifying power of one hundred, we should need ten thousand times the light. I do not mean that this quantity of light is always necessary; it is not so, because we can commonly see an object with less than its natural illumination. Still, we need a certain amount of light, or it will be too dim.

In order that distinct vision of a distant object may be secured in the telescope, the one great essential is that the object-glass should bring all the rays coming from any one point of the object observed to the same focus. If this is not brought about; if different rays come to slightly different foci, then the object will look blurred, as if it were seen through a pair of spectacles which did not suit our eyes. Now, a single lens, no matter of what sort of glass we make it, will not bring rays to the same focus. The reader is doubtless aware that ordinary light, whether coming from the sun or a star, is of a countless multitude of different colours, which can be separated by passing the light through a triangular prism. These colours range from red at one end of the scale, through yellow, green, and blue, to violet at the other. A single lens brings these different rays to different foci; the red farthest from the object-glass; the violet nearest to it. This separation of the rays is called dispersion.

The astronomers of two centuries ago found it impossible to avoid the dispersion of a lens. About 1750, Dollond, of London, found that it was possible to correct this defect by using two different kinds of glass, the one crown glass and the other flint glass. The prin-