ABSTRACT
The light by which we see the Sun, Moon, planets, stars and even the furthermost galaxies all comes through a very narrow range in the electromagnetic spectrum, extending from approximately 3,900 Å in the violet to 7,400 A in the red (Fig. 35). These limits depend upon the sensitivity of the human eye, which varies appreciably from one individual to another. As long ago as 1800, however, Sir William Herschel used a thermometer with a blackened bulb to show that there exist other, invisible, radiations in the spectrum of the Sun, lying beyond the red end of the visible range, which produce a heating effect. These are known as the infra-red radiations, and by means of special photographic plates it is possible to trace them out to about 12,000 Å. Other means enable us to detect them to the limit of the solar spectrum as far as 50,000 A. Radiation of even longer wavelengths lies beyond the infra-red, these being known as the Hertzian or radio waves.
