Page:Williamherschel00simegoog.djvu/168

156 an unknown centre. He did not find it, but he believed, as we have already seen, first, that the sun was moving among the stars, and second, that it was moving towards a spot in the constellation Hercules in the northern sky.

As the sun is the source of light and heat, and both of them had to be considered in his observations, it was natural that Herschel should turn his thoughts to the solar spectrum, as we call what is commonly spoken of as the rainbow. A glass prism produces the same effect on a beam of sunlight as a raindrop or a cloud curtain composed of millions of them: it divides or decomposes the white light of one sun into that of seven suns of different colours, red, orange, yellow, green, blue, indigo, violet, and it also bends or refracts them from the straight line the sunbeam would otherwise pursue. The red is the least bent, the violet most. By the refraction or bending is meant what is seen by thrusting one half of a walking-stick into water, and keeping the other half out of it in the air. But it happened that in shielding his eye from the sun when looking at its disc through a telescope, Herschel had used glass of various colours to dim the glare and heat. This experience was fatal to the use of glass coloured red. "I began with a red glass," he says, "and, not finding it to stop light enough, took two of them together. These intercepted full as much light as was necessary; but I soon found that the eye could not bear the irritation, from a sensation of heat, which, it appeared, these glasses did not stop. I now took two green glasses: but found that they did not intercept light enough. I therefore smoked one of them: and it appeared that, notwithstanding they