206 PROGRESS OF SCIENCE. 



sphere is the Corona^ whose nature is not yet determined.* 

 We may state that the sun has 700 times more matter than 

 all the planets put together. Herschel discovered (March 

 13, 1781) the planet Uranus and its six moons with their 

 westerly motion ; he measured the diameter of several 

 planets (Ceres, Juno, Pallas, Vesta) ; he ascribed the streaks 

 of Jupiter and Saturn to be due to clouds driven by regular 

 winds, resembling our trade-winds. He further observed a 

 volcano in activity in the moon, in April, 1787. But Her- 

 schel is to be remembered as having made three capital dis- 

 coveries : I. that binary stars are very numerous ; II. that 

 nebulae are existing throughout the heavens in thousands ; 

 III. that the Newtonian law of GRAVITATION IS A UNIVERSAL 

 LAW. As a physicist, he made CATOPTRIC TELESCOPES 

 (telescopes by reflection in contradistinction to dioptric tele- 

 scopes i.e., by refraction) ; he also made numerous observa- 

 tions of great value. He was the first to study the SOLAR 

 SPECTRUM (with the thermometer, 1800), and thereby caused 

 immense progress in optics nothing less than the opening of 

 a new area of researches. For, wishing to ascertain with a 

 thermometer which of the coloured rays given by the prism 

 were the hottest, he discovered HEAT-RAYS that is, rays of light 

 which are invisible to us beyond the red band of the spectrum. 

 Within a year RITTER (J. W T ., 1776 1810) discovered DARK 

 RAYS at the other extremity of the spectrum, beyond the 

 violet band, which have the property of decomposing nitrate 

 of silver and other compounds, for which reason they have 

 been called CHEMICAL RAYS. Herschel's was the first ex- 

 periment in the history of spectrum analysis. (See Fraunhofer 

 and Kirchhoff.) The discovery made by Ritter was, next to 

 Porta's invention of the camera obscura, the earliest of 

 those which led to photography : we have seen that Davy, in 

 1802, tried to utilise Ritter's discovery. Herschel's magni- 

 ficent labours were worthily continued by his son. 



1746 1826. Piazzi, following the indications furnished 

 by Kepler's empirical law by which the planets' distances 



* Mr. Taylor has calculated that the whole outward envelope of the 

 sun cannot extend to less than 800,000 miles, and the light we receive from, 

 the centre must pass through this vast ocean of matter. 



