[ 155 ] 

 XXIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 66.} 



May 18, "/^\^ a new case °^ ^ e I ntei *fe rence °f Light-" By 

 1848. " the Rev. Baden Powell, M.A., F.R.S. &c, Savilian 

 Professor of Geometry in the University of Oxford. 



The principal experiment exhibiting the newly-observed case of 

 interference described by the author, is the following : into a hollow 

 prism or trough containing oil of sassafras, anise, cassia, &c, a plate 

 of glass is dipped so as to intercept the light passing through the 

 upper or thicker half of the prism, and leave the lower half clear : 

 the spectrum of a line of light or narrow slit, is then immediately 

 seen to be covered with dark bands parallel to the sl't or edge of the 

 prism. 



Some substances, as oil of turpentine, water, &c, give no bands 

 with this arrangement, but will give them, if a complementary 

 arrangement be adopted, viz. by placing a narrow slip of glass at 

 the bottom of the prism, or so as to intercept the light passing 

 through the thinner half. 



The production of the bands in general, and the reason of the dif- 

 ference existing in the cases referred to, are fully accounted for on the 

 theory of waves and interference : whence the following formula is 

 deduced, giving the number of bands (n), when fi p and fx m , the in- 

 dices for any ray for the plate and for the medium, are known, of 

 which X is the wave length ; and taking these quantities for any two 

 rays (1) and (2) of the spectrum, r being the thickness of the plate, 

 we have the number of bands between them expressed by 



°={(^)r(^)> 



The numbers thus calculated agree with observation for many 

 media. The distinction of the two cases alluded to depends on 

 whether n result positive or negative •■ that is, on the relative values 

 of the indices ; also agreeing with observation. 



If doubly refracting media are used as plates, two sets of bands 

 are seen superimposed, which are easily shown to be due to the or- 

 dinary and extraordinary rays respectively. But for some of these 

 crystals the data are as yet insufficient to give more than a general 

 accordance with theory. 



The method might be applied practically for determining the in- 

 dices of many substances to which the ordinary method is inapplica- 

 ble from the impossibility of forming them into prisms : n will also 

 exhibit palpably the most insensible degree of double refraction, and 

 may thus become useful to the mineralogist. 



There is a close analogy between these phenomena and those 

 observed by Baron von Wrede, and by Sir D. Brewster and Mr. 

 Fox Talbot, of which Mr. Airy has given a theory. A similar 

 theory is necessary for explaining some of the more minute details 



