must be so lost was evident, but no data existed for determining the loss. Herschel was thus led to a long series of photometric experiments on the reflecting powers of the metals used in his mirrors, and on the amount of light transmitted by lenses. Applying the corrections thus deduced experimentally, he found that the space-penetrating power of his twenty-foot telescope, with which he made his star-gauges, was sixty-one times that of the unassisted eye, while the space-penetrating power of his great forty-foot telescope was one hundred and ninety-two times that of the eye. In support of his important conclusions Herschel had an almost unlimited amount of experimental data in the records of his observations, of which he made effective use.
By far the most important of Herschel's work in the domain of pure physics was published in the same year (1800), and related to radiant heat. The investigation of the space-penetrating powers of telescopes was undertaken for the sole purpose of aiding him in measuring the dimensions of the stellar
