easily distinguished at a mile distance, when the rays of the sun shine on him, and by suppressing the intermediate light which surrounds us, and darkening our sight, we should see him at least ten times farther; that is to say, ten miles. Ships, therefore, being much larger, would be seen as far as the curvature of the earth would permit, without any other instrument than the naked eye. [H] Aristotle is, I believe, the first that ever mentioned this observation. But a concave mirror, of a great diameter, and of any focus, placed at the end of a long black tube, would have nearly the same effect as our great objective glasses of the same diameter and form would have during the night, and it was probably one of these concave mirrors of polished steel that was established at the port of Alexandria[I]. If this steel mirror did really exist, we cannot refuse to the ancients the glory of the first invention, for this mirror can only be effective by as much as the light reflected by its surface was collected by another concave mirror placed at its focus, and in this consists the essence of the telescope and the merit of its construction. Nevertheless this does not deprive the great Newton of any glory, who first renewed the almost-forgotten invention. As the rays of light are by their nature differently refrangible, he was inclined to think there were no means of correcting this effect, or, if he had perceived those means, he judged them so difficult that he chose rather to turn his views another way, and produce, by means of the reflection of the rays, the great effects which he could not obtain by their refraction; he, therefore, constructed his telescope, the reflection of which is infinitely superior to those that were in common use. The best telescopes are always dark in comparison of the acromatic, and this obscurity does not proceed only from the defect of the polish, or the colour of the metal of mirrors, but from the nature even of light, the rays of which being differently refrangible are also differently reflexible, although in much less unequal degrees. [I] From time immemorial the Chinese, and particularly the Japanese, have possessed the art of working in steel both in large and small bodies; and hence I have thought that the words _e ferro sinico_ in the preceding quotation should be understood as applying to polished steel. It still remains, therefore, to bring the telescope to perfection, and to find the manner of compensating this different reflexibility, as we have discovered that of compensating the different refrangibility.