DISCOVERY BY MEANS OF NEW MODES OF OBSERVATION. 573 



number of bands in various spectra. By the use also of 

 electrometers and galvanometers of increased delicacy, new 

 sources of free electricity and of electric currents have 

 frequently been revealed. 



It was by means of astronomical instruments of greatly 

 increased accuracy that Tycho Brahe, in the latter part of 

 the sixteenth century, was enabled to make an immense 

 number of new observations. Galileo, in 1611, by em- 

 ploying a new instrument (the telescope) discovered the 

 moons of Jupiter, but in consequence of the defective 

 power or insufficient delicacy of the instrument, mistook 

 Saturn's rings for two stars; and Huyghens, in 1659, by 

 employing more accurate lenses, discovered the ring and 

 one of the satellites of that planet. It was by means of 

 the more accurate measurement and observation of an arc 

 of the meridian in France, by Picard, in the year 1670, 

 that Newton was enabled to verify the hypothesis and 

 discover the law of universal gravitation in its action with 

 regard to our moon. 



Savart, by adopting Lichtenberg's device of strewing 

 powders upon plates, discovered ' that the vibrations arti- 

 fically produced in a plate of bi-axial crystal, indicated 

 the existence of varying elasticity in varying directions.' 1 

 Eitter, by employing the novel plan of using papers wetted 

 with a solution of nitrate of silver, and allowing the solar 

 spectrum to fall upon it in the dark, in the year 1801, 

 discovered the dark chemical rays in solar beams. Wol- 

 laston's reflecting goniometer also, being a great improve- 

 ment upon that of Rome de Lisle, the measurements 

 obtained by it being more accurate the smaller the faces 

 of the crystal (because the angles were measured by means 

 of the reflected images of bright objects seen in them), 

 enabled many new truths to be discovered which could not 

 1 Jevons, Principles of Science, vol. ii. p. 364. 



