A HALF-CENTURY OF SCIENCE. 



199 



haps, just mention, as regards telescopes, that the largest reflector, in 

 1830, was Sir W. Herschel's, of four feet ; the largest at present be- 

 ing Lord Rosse's, of six feet ; as regards refractors, the largest then 

 had a diameter of eleven and a quarter inches, while your fellow- 

 townsman, Cooke, carried the size to twenty-five inches, and 31 r. 

 Grubb, of Dublin, has just successfully completed one of twenty-seven 

 inches for the Observatory of Vienna. It is remarkable that the two 

 largest telescopes in the world should both he Irish. 



The general result of astronomical researches has been thus elo- 

 quently summed up by Proctor : " The sidereal system is altogether 

 more complicated and more varied in structure than has hitherto been 

 supposed ; in the same region of the stellar depths coexist stars of 

 many orders of real magnitude ; all orders of nebula?, gaseous or 

 stellar, planetary, ring-formed, elliptical, and spiral, exist within the 

 limits of the galaxy ; and, lastly, the whole system is alive with move- 

 ments, the laws of which may one day be recognized, though at pres- 

 ent they appear too complex to be understood." 



"We can, I think, scarcely claim the establishment of the undula- 

 tory theory of light as falling within the last fifty years ; for though 

 Brewster, in his " Report on Optics," published in our first volume, 

 treats the question as open, and expi'esses himself still unconvinced, 

 he was, I believe, almost alone in his preference for the emission 

 theory. The phenomena of interference, in fact, left hardly any if 

 any room for doubt, and the subject was finally set at rest by Fou- 

 cault's celebrated experiments in 1850. According to the undulatory 

 theory, the velocity of light ought to be greater in air than in water, 

 while if the emission theory were correct the reverse would be the 

 case. The velocity of light 186,000 miles in a second is, however, 

 so great that, to determine its rate in air, as compared with that in 

 water, might seem almost hopeless. The velocity in air was, never- 

 theless, determined by Fizeau in 1849, by means of a rapidly revolv- 

 ing wheel. In the following year Foucault, by means of a revolving 

 mirror, demonstrated that the velocity of light is greater in air than 

 in water thus completing the evidence in favor of the undulatory 

 theory of light. 



The idea is now gaining ground that, as maintained by Clerk 

 Maxwell, light itself is an electro-magnetic disturbance, the luminifer- 

 ous ether being the vehicle of both light and electricity. 



AVunsch, as long ago as 1T92, had clearly shown that the three 

 primary colors were red, green, and violet ; but his results attracted 

 little notice, and the general view used to be that there were seven 

 principal colors red, orange, yellow, green, blue, indigo, and violet ; 

 four of which namely, orange, green, indigo, and violet were con- 

 sidered to arise from mixtures of the other three. Red, yellow, and blue 

 were therefore called the primary colors, and it was supposed that in 

 order to produce white light these three colors must always be present. 



