On Achromatic Interference-Bands, 189 



about — 02 mm.; while Wullner* found for a (non-saturated) 

 15-per-cent. solution of sodium sulphate at 26°*3, a vapour- 

 pressure 1*2 mm. less than that of pure water. Hence it 

 seems that, just at the point where the solution becomes fully 

 saturated, there is a discontinuity in its vapour-pressure. 



According to Regnault's classical researches, a vapour- 

 pressure of 10*7 mm. over pure water corresponds to a tem- 

 perature of 12°-40 (instead of 12°'72). The influence of the 

 parchment-paper wick remains to be determined. Bat even 

 should it produce no increase in the vapour-pressure, there 

 would still be, on the whole, a considerable gain of motivity in 

 experiment III. 



In all these experiments the working substance becomes 

 separated into two portions, which are not identical in che- 

 mical composition. I would therefore suggest this restric- 

 tion, that ice cannot as yet assume with certainty the truth of 

 Carnot's Principle when chemical separation occurs between two 

 finite portions of the toorking substance. Further research 

 seems necessary before we can say whether or not this is the 

 only exception to the truth of Carnot's Principle. 



XXVIII. On Achromatic Interference-Bands. By Lord 

 Bayleigh, Sec. JR.S., Professor of Natural Philosophy in 

 the Royal Institution. 



[Concluded from p. 91.] 



Airy's Theory of the White Centre. 



IF a system of interference-bands be examined through a 

 prism, the central white band undergoes an abnormal dis- 

 placement, which has been supposed to be inconsistent with 

 theory. The explanation has been shown by Airy f to depend 

 upon the peculiar manner in which the white band is in 

 general formed. Thus, " Any one of the kinds of homoge- 

 neous light composing the incident heterogeneous light will 

 produce a series of bright and dark bars, unlimited in number 

 so far as the mixture of light from the two pencils extends, 

 and undistinguishable in quality. The consideration, there- 

 fore, of homogeneous light will never enable us to determine 

 which is the point that the eye immediately turns to as the 

 centre of the fringes. What, then, is the physical circumstance 

 that determines the centre of the fringes ? 



" The answer is very easy. For different colours the bars 



* Pogg. Ann. ciii. p. 543. 



+ Airy, " Remarks on Mr. Potter's Experiment on Interference," 

 Phil. Mag. ii. p. 161 (1833). 



