Vlll I'UKKACK. 



definitely proposed by tbe youuger vou llelinboltz' and long before the era <>t' the 

 X-ray, tbis polioy seemed agaiu to devolve iipnu me. .My discussions may therefore 

 appear to proceed from a point of view since sii[)eiseded, but I jjelieve the} aie con- 

 sistent and fair. Hence if the light of this volume is still the aMti([uated tallow dip, 

 the number of interesting results to which it has guided, encourages me to hope 

 that the quest, apart from any theory, may have been worth the caudle. 



C. Bakus. 

 Bkown Univeksitv, Pkovidenck, R. I., 



Aujrust, 1902. 



' R. von Helmholtz: ll'in/. Ann., x.wii, p. 508, 1886; iliiJ., xxxii, j). i, 1887. Cf. j). 2. 



