OPTICS.
THE invention of the telescope and the microscope, the discoveries made concern ing the properties of light and the laws of vision, added to the facility of applying mathematical reasoning as an instrument of investigation, had long given a peculiar interest to optical researches. The experiments and inquiries of Newton on that sub ject began in 1666, and soon made a vast addition both to the extent and importance of the science. He was at that time little more than twenty-three years old ; he had al ready made some of the greatest and most original discoveries in the pure mathema tics; and the same young man, whom we have been admiring as the most profound and inventive of geometers, is to appear, almost at the same moment, most pa tient, faithful, and sagacious 'interpreter of nature. These characters, though certain ly not opposed to one another, are not often combined; but to be combined in so high a degree, and in such early life, was hitherto without example.
In hopes of improving the telescope, by giving to the glasses a figure different from the spherical, he had begun to make experiments, and had procured a glass prism, in order, as he tells us, to try with it the celebrated phenomena of colours.' These trials led to the discovery of the different refrangibility of the rays of light, and are now too well known to stand in need of a particular description.
Having admitted a beam of light into a dark chamber, through a hole in the win dow-shutter, and made it fall on a glass prism, so placed as to cast it on the oppo site wall, he was delighted to observe the brilliant colouring of the sun's image, and not less surprised to observe its figure, which, instead of being circular, as he ex pected, was oblong in the direction perpendicular to the edges of the prism, so as to have the shape of a parallelogram, rounded at the two ends, and nearly five times as long as it was broad.
When he reflected on these appearances, he saw nothing that could explain the elongation of the image but the supposition that some of the rays of light, in pas sing through the prism, were more refracted than others, so that rays which were parallel when they fell on the prism, diverged from one another after refraction, the rays that differed in refrangibility differing also in colour. • The spectrum, or
solar image, would thus consist of a series of circular images, partly covering one another, and partly projecting one beyond another, from the red or least refrangible rays, in succession, to the orange, yellow, green, blue, indigo, and violet, the most re frangible of all.
It was not, however, till he tried every other hypothesis which suggested itself to his mind by the test of experiment, and proved its fallacy, that he adopted this as a true interpretation of the phenomena. Even after these rejections, his explanation had still to abide the sentence of an experimentum cruc is.
Having admitted the light and applied a prism as before, he received the coloured spectrum on a board at the distance of about twelve feet from the first, and also pierced with a small hole. The coloured light which passed through this second hole was made to fall on a prism, and afterwards received on the opposite wall. It was then found that the rays which had been most refracted, or most bent from their course by the first prism, were most refracted also by the second, though no new colours were produced. " So," says he, " the true cause of the length of the image was detected to be no other ' than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall."' It was also observed, that when the rays which fell on the second prism were all of the same colour, the image formed by refraction was truly circular, and of the same colour with the incident light. This is one of the most conclusive and satisfactory of all the experiments.