274 THOMAS YOUNO. 



With enormous labour he calculated the positions where 

 the phenomena of interference must display themselves 

 in a definite way. He was, moreover, a most refined 

 experimentalist, and having made his calculations, he 

 devised instrumental means of the most exquisite deli- 

 cacy with the vie^f of verifying his results. In this 

 way he swept the field of diffraction practically clear of 

 difficulty, solving its problems where even Young had 

 failed. 



Truly, these were minds possessing gifts not pur- 

 chasable with money! And round about the central 

 labours of each, minor achievements of genius are to be 

 found, which would be a fortune to less opulent men. 

 I hardly know a more striking example of Young's pene- 

 tration than his account of the spurious or supernume- 

 rary bows observed within the true primary rainbow. 

 These interior bows are produced by interference. It 

 is not difficult, by artificial means, to form them in 

 great number and beauty. This is a subject on which 

 I worked assiduously a couple of years ago. 1 And often, 

 when looking at these wondrous interference circles, 

 the words of Young seemed to me like the words of 

 prophecy. The bows were the physical transcript of 

 what he stated must occur ; a transcript, moreover, 

 which, compared with his words, was far more complete 

 and impressive than any ever exhibited by the rainbow 

 in Nature. Take another instance. The beautiful rings 

 of colour observed when a point of light is looked at 

 through the seeds oi lycopodium shaken over a piece 

 of glass, or shaken as a cloud in the air, are known to 

 be produced by minute particles all of the same size. 

 The iridescence of clouds seen sometimes in great 

 splendour in the Isle of Wight, but more frequently in 

 the Alps, is due to this equality in the size of the 



1 Sec art. ' The Rainbow and its Congeners,' in this volume. 



