Page:Astronomy for Everybody.djvu/74

ciple by which this is done is very simple. Crown glass has nearly the same refracting power as flint, but it has nearly twice the dispersive power. So Dollond made an objective of two lenses, a section of which is shown in the figure. First there was a convex lens of crown glass, which is of the usual construction. Combined with this is a concave lens of flint glass. These two lenses, being of opposite curvatures, act on the light in opposite directions. The crown glass tends to bring the light to a focus, while the flint, being concave, would make the rays diverse.
Fig. 10.—Section of the Object-glass of a Telescope. If it were used alone, we should find that the rays passing through it, instead of coming to a focus, diverge farther and farther from a focus, in different directions. Now, the flint glass is made with but little more than half the power of the crown. This half power is sufficient to neutralize the dispersion of the crown; but it does not neutralize much more than half the refraction. The combined result is that all the rays passing through the combination are brought nearly to one focus, which is about twice as far away as the focus of the crown alone.

I say brought nearly to one focus. It happens, unfortunately, that the combined action of the two glasses is such that it is impossible to bring all the rays of the various colours absolutely to the same focus. The divergence, in the case of the brighter rays, can be made very small indeed, but it cannot be cured entirely. The larger the telescope, the more serious the defect. If you look