478 SPECIAL METHODS OF DISCOVERT. 



appliances, I beg to refer the reader to Whewell's 

 1 History of the Inductive Sciences,' vol. ii., 3rd edition, 

 p. 207. 



Wollaston, by inventing his reflecting goniometer, 

 rendered a great aid to mineralogists, enabled many dis- 

 coveries to be made in the science of crystallography, and 

 conduced largely to its subsequent great improvement. It 

 was by means of a prism of colourless glass that Descartes 

 first showed that a beam of white light is spread out into 

 a spectrum, possessing all the colours of the rainbow ; and 

 Newton, by passing rays of different colours successively, 

 in the same line through such a prism, discovered that 

 each differently-coloured ray was differently refracted. 

 The discoveries of the polarising properties of Iceland- 

 spar, tourmaline, and bundles of sheet-glass, led to the 

 invention of a variety of polariscopes, and by means of 

 them to many discoveries in optical science. Biot, by 

 examining liquids in long tubes with polarised light, 

 discovered that some of them possessed the property 

 known as circular polarisation ; and therefore that crys- 

 talline structure was not a necessary condition of that 

 property. Seebeck made, independently, the same dis- 

 covery. By the use of the spectroscope, invented by 

 Fraunhofer, Kirchoff, and others, no less than five new 

 metals, viz. caesium, rubidium, thallium, indium, and 

 gallium, have been found, and a whole host of disco- 

 veries have been made respecting the composition of the 

 sun and other heavenly bodies. It was by its assistance 

 that Miller and Huggins, in 1862, discovered that the 

 composition of the atmosphere of Jupiter was partly 

 like our own, also that Mars and the rings of Saturn 

 have atmospheres not much unlike ours. By similar 

 means they ascertained the composition of the stars 

 Aldebaran, Betelgeux in Orion, and /3 Pegasi, and that 



