386 W. LeConte Stevens—Lecent Progress in Opties. 
momentary impression by white light becomes analyzed into 
the subjective impression of spectral colors, the duration of 
the retinal sensation varying with the wave length of the inci- 
dent light. The law of this variation has been studied by 
Plateau,* Nicholst and more recently with much precision by 
Ferry,{ who showed that retinal persistence varies inversely 
as the logarithm of the luminosity. For a given source of 
light separated into its spectral components, the yellow is the 
brightest. For this hue accordingly the retinal impression is 
shortest and for violet it is longest. 
Under appropriate conditions the after-effect on the retina 
has a certain pulsatory character, as first noted by C. A. Young§ 
in 1872, and carefully studied within the last few years by 
Charpentier| in France and Shelford Bidwell§[ in England. 
A disk with properly arranged black and white sectors, if 
brightly illuminated and looked at while revolving at a mode- 
rate rate, becomes apparently colored, just as a momentary 
glance at the sun causes the perception of a succession of sub- 
jective spectral hues which may last a number of seconds. The 
phenomenon in relation to the disk was known as early as 
1838** and explained by Roodtt in 1860. The re-discovery of 
what has been long forgotten arouses all the interest of novelty. 
The “artificial spectrum top” devised by Benham{t¢t last autumn 
excited interest on two continents, and was promptly copy- 
righted by a prominent firm of opticians§§ 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 abundantly 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 abund- 
ance of her reward. 
* Dissertation sur quelques propriétés des impressions produits par la lumiére 
sur lorgane de la vue, Liege, 1829. 
+ This Journal, October, 1884. 
Ibid., Sept., 1892. 
Philosophical Magazine, vol. xliii, p. 343, 187 oe 
‘ Oscillations rétiniennes,’”’ Comptes Rendus, vol. exiii, p. 147, 1891. 
On the Recurrent Images following Visual Impressions, Proc, Royal Society, 
March 27, 1894. 
** Fechner, Poggendorff’s Annalen, 1838, 
tt This Journal, September, 1860. 
tt Nature, Nov. 29, 1894, p. 113. 
§§ Ibid., March 14, 1895, p. 463. 
