Prof. W. Gibbs on the Measurement of Wave-lengths, 177 



magnetic changes in bars of iron, steel, nickel, &c, especially if 

 a powerful battery and a delicate galvanometer are employed, 

 and might probably be used to throw a light upon the influence 

 which the presence of foreign substances have upon iron &c. 



The foregoing phenomena may be employed as an illustration 

 of a very general (I may say, universal) property of matter 

 which has not, that I am aware, been specially recognized as 

 such. Every substance, even those of the simplest constitution 

 (as the elementary bodies, and even those of them which are in 

 the gaseous state), when acted upon by a single external force, pos- 

 sesses the power of dividing the influence of that force in such a 

 way that, instead of producing only one force or one effect, it 

 produces several ; or, stated more briefly, matter has a universal 

 property of dividing and multiplying forces and effects. For 

 instance, simply in heating a bar of iron to redness a whole 

 series of changes occur in its molecular structure, its magnetism, 

 its dimensions, and its cohesive power, in addition to the changes 

 in its specific heat, its thermoelectric capacity, and its electric 

 conducting-power. The changes produced by heat in even so 

 simple a substance as iron were so numerous in some of these 

 experiments as to produce the impression that the metal was 

 endowed with vitality. 



XXI. On the Measurement of Wave-lengths by means of Indices 

 of Refraction. By Wolcott Gibbs, M.D., Rumford Pro- 

 fessor in Harvard University*. 



IN a brief notice f communicated to the British Association 

 for the Advancement of Science at its Meeting in 1849, 

 Professor Stokes has given a method for measuring wave-lengths, 

 which depends upon the fact that, in substances of medium re- 

 fractive power, the increment of the index of refraction in passing 

 from one point of the spectrum to another is nearly proportional 

 to the increment of the square of the reciprocal of the wave- 

 length. The author showed that even when the intervals were 

 taken much longer than necessary, the error in the wave-length 

 was usually only in the eighth place of decimals. At the date 

 of the publication of this notice the subject of wave-lengths 

 possessed but little interest. The recent development of the 

 spectral analysis of light has given a new impulse to this branch 

 of optics, and has rendered necessary the construction of a 

 normal map of the entire solar spectrum. This has been most 



* From Silliman's American Journal for July 18/0. Read before the 

 National Academy of Sciences, April 12, 1870. 



f Report of the British Association for the Advancement of Science for 

 1849. Notices and Abstracts, p. 10. 



