186 PROGRESS OF SCIENCE IN THE CENTURY. 



finest illustrations of the stability of the gravitational 

 theory. 



Comets. — Another series of confirmations of New- 

 tonian laws is concerned with comets. For, although 

 Newton had shown that their movements were in har- 

 mony with his general formula, he had few data at 

 his command, and a clearer demonstration was given 

 by Halley, who, from a basis of calculations, accu- 

 rately predicted the return of " Halley's comet " in 

 1758-9. 



The physician Olbers (d. 1840) introduced a sim- 

 plification in the method of computing the paths of 

 comets, and for half a century was one of the most 

 assiduous and successful students of these periodic 

 visitants. Among the many whom he helped and 

 stimulated during his long life was Encke, a pupil 

 of Gauss, one of those who have passed through the 

 portal of mathematics to the study of the stars. 

 Sixty-three years after Halley's prediction was veri- 

 fied, Encke in 1822 had a similar success with a 

 comet ^^ of short period," which revolves round the 

 sun in about three and a quarter years. 



More than 200 comets have been studied in the 

 nineteenth century; and by means of the spectro- 

 scope, applied to the study of comets by Donati in 

 1864 and by Huggins in 1868, it has been possible 

 to advance a little beyond the computation of paths 

 and periods, and to prove, for instance, that at least 

 some comets are in part self-luminous, while others, 

 especially those of short period, appear to owe most 

 of their brilliance to light reflected from the sun. 

 Professor Tait seems to have been the first to give 

 definite expression to the idea (expounded by Lord 

 Kelvin in 1871) that the light of comets, and of 

 nebulae as well, may be due to flashes of ignited gas 



