134 RECENT PROGRESS IN OPTICS. 



Bidwell ("On the recurrent images following visual impressions," Proc. 

 Eoyal Society, March 27, 1894) in England. A disk with properly 

 arranged black and white sectors, if brightly illuminated and looked at 

 while revolving at a moderate rate, becomes apparently colored, just as 

 a momentary glance at the sun causes the perception of a succession 

 of subjective spectral hues which may last a number of seconds. The 

 phenomenon in relation to the disk was known as early as 1838 (Fech- 

 uer, Poggendorff's Annaleu, 1838), and explained by Eood (American 

 Journal of Science, September, 1860) in 1860. The rediscovery of 

 what has been long forgotten arouses all the interest of novelty. The 

 "artificial spectrum top," devised by Benham (Nature, November 29, 

 1894, p. 113) last autumn, excited interest on two continents, and was 

 promptly copyrighted by a prominent firm of opticians (ibid., March 

 14, 1895, p. 463) in England. It would perhaps be equally enterprising 

 to copyright the solar spectrum. 



The limits of a single address forbid my touching upon the large 

 and practically important subject of color blindness. Indeed, in both 

 physical and physiological optics much has been omitted that is abun 

 dautly worthy of attention. In behalf of my hearers it may be wise to 

 take heed, once more, of the fate of Tarpeia, who was overwhelmed 

 with the abundance of her reward. 



