The astronomer has to enlarge his object-glass, precisely for the same reason that the owl expands its eye in the dark. The expanded eye does not magnify objects; it merely takes in more light, and sees more clearly. The mouse is large enough to be seen by the owl with contracted eye, but it is too indistinct to be visible; the pupil is therefore expanded, and the prey, though retaining the same apparent dimensions, is at once descried. So the astronomer often fails to descry objects, not because they are not large enough, but because they are not clear enough; and the only way of overcoming the difficulty, is by making his artificial pupil, namely, the object-glass, as large as possible.
M. Steinheil of Munich has, at last, succeeded in working lenses, according to the formula of Gauss, the effect of which will be to shorten greatly the length of telescopes. In the best achromatic telescopes, there is an outstanding aberration which cannot be compensated, and which increases with the size of the object-glass. To lessen the injurious effect, the telescope must be lengthened in a certain proportion, as the lens is- increased in size; but in M. Steinheil's plan, this proportion is much less than in ordinary telescopes. It is impossible to over-estimate the advantage of this in practical astronomy. Every advance in power is at present neutralised, in a great measure, by the cumbrous dimensions every increase in the size of the lens involves. The mere unwieldiness is a great drawback to accuracy, while
