Sir W. Herschel appears to have been the first who made a distinction between the _magnifying_ power, and the _space-penetrating_ power of a telescope; and there are many examples which prove that such a distinction ought to be made, especially in the case of large instruments. For example, the small star, or speck of light, which accompanies the pole-star, may be seen through a telescope of large aperture, with a smaller magnifying power than with a telescope of a small aperture furnished with a much higher power. If the magnifying power is sufficient to show the small star completely separated from the rays which surround the large one, this is sufficient in one point of view; but in order that this effect may be produced, so as to render the small star perfectly distinguishable, a certain quantity of light must be admitted into the pupil of the eye--which quantity depends upon the area of the object-glass or speculum of the instrument, or, in other words, on the illuminating power. If we compare a telescope of 2-3/4 inches aperture with one of 5 inches aperture, when the magnifying power of each does not exceed 50 times for terrestrial objects, the effect of illuminating power is not so evident; but if we use a power of 100 for day objects, and 180 for the heavenly bodies, the effects of illuminating power is so clearly perceptible, that objects not only appear brighter, and more clearly visible, in the larger telescope, but with the same magnifying power, they also _appear_ larger, particularly when the satellites of Jupiter and small stars are the objects we are viewing.