318 Lord Rajleigli on t/ie Lower Limit 



allow us to enter into theoretical discussions concerning tLe 

 structural formula of camphor and the substances related to it; 

 nor can wo venture to make any generalizations concerning 

 the members of the terpene group. Both of these subjects 

 are of much interest. We purpose to prepare much larger 

 quantities of the terpenes, sal viol, and sage-camphor, and to 

 determine the pJiysical and chemical properties of these bodies, 

 combinino" this investio-ation with a research into the consti- 

 tution of the camphors in general. 



^\'^e take this opportunity of again returning our best 

 thanks to Drs. Gladstone and Tilden for tlieir kindness in 

 examing the optical and other properties of the two sage- 

 terpenes. 



XL VII. On the Lower Limit of the Prismatic Spectrum^ ivith 

 especial reference to some Observations of Sir John Herschel. 

 By Lord "Rayleigh, FM.S.* 



ALL theoretical investigations into the laws of dispersion 

 have led to the conclusion that, as the wave-length of 

 light increases, the refractive index for a given substance ap- 

 proaches a hnite limit, below which it cannot pass. According 

 to Cauchy, the refractive index (yct) can be expressed approxi- 

 mately as the sum of two tei-ms ; of which the first is constant 

 and coincident with the limiting value just spoken of, while 

 the second \'aries inversely as the square of the wave-length 

 (X). This law, which may be expressed fj.-- A + B\~^, agrees 

 very well with observation over the less-refrangible part of the 

 spectrum, and may fairly be extended to the ultra-red rays ; 

 so that, apart from all theory, there is good reason for thinking 

 that fi cannot fall below a certain value, however great the 

 wave-length may be. 



In estimating the place of the limit on any particular map 

 of the spectrum, we must bear in mind that in ordinary pris- 

 matic spectra equal distances do not correspond to equal dif- 

 ferences of fi. The prismatic spectrum as seen at one view 

 through a prism of considerable angle depends, not only upon 

 the material and angle of the prism, but also upon the actual 

 position in which the prism is placed. Thus different spectra 

 are obtained according to the ray selected for minimum devia- 

 tion : if the green ray pass Avith minimum deviation, the de- 

 viations of the extreme rays of the spectrum are greater than 

 if the prism were set specially for them, and thus the red end 

 of the spectrum is unduly compressed and the blue end is un- 



* Comniimicated by tlie Author, having "been read iu suhstance before 

 the British Association at Plymouth, August 1877. 



