52 THE INTERFERENCE SPECTRUM. 



CHAPTER V. 



ON THE INTERFERENCE SPECTRUM. 



CONTENTS : Defects of the Prismatic Spectrum. Mode of forming the Interference 

 Spectrum. Its Peculiarities. The Distribution of the Colours, and Law of their In- 

 tensities. Reflected Interference Spectrum. Its Fixed Lines. Wave-lengths of the 

 Seven Great Rays. 



Melloni's Researches on the Distribution of Heat in Perfect Prismatic Spectra. Ap- 

 parent Identity of Light and Heat. 



Distribution of Chemical Force in the Interference Spectrum. Comparison of the Fix- 

 ed Lines in the Prismatic and Interference Spectrum. Mode of Defining Chemical 

 Effects by Wave-lengths or by Times of Vibration. Impression on Bromide of Silver. 

 On Chloride of Silver. Total Change in the Distribution of Heat in the Inter- 

 ference Spectrum. 



181. THE prismatic spectrum is obtained in a state of the greatest purity when the 

 decomposed beam of light comes through a narrow fissure, and the instrumental ar- 

 rangement given in (153) is employed. All the fixed lines are then thrown upon the 

 screen, and the separation of the different homogeneous rays is accomplished, perhaps, 

 as perfectly as is possible. 



182. But in this spectrum, as employed for investigating the chemical action of the 

 solar beam, very serious difficulties may be traced. If we inspect a photographic im- 

 pression, or if we consider the intensity of the luminous rays in its different parts, we 

 perceive that, as the violet end is approached, the different changes take place in a less 

 abrupt manner ; the light fades gradually and imperceptibly away, the photograph does 

 not end sharply, but, spreading itself out to a great distance, disappears so gradually 

 that we can scarcely say where its termination is, and the fixed lines increase in num- 

 ber and breadth compared with what we perceive at the red extremity. 



183. These different results obviously arise from an inherent defect in the prismatic 

 spectrum ; a defect which originates in the very cause which gives rise to the spectrum 

 itself unequal refrangibility. Of two sets of rays compared together, one set taken in 

 the red, and the other in the violet region, it is obvious that, in the same spectrum, 

 from the very circumstance of their greater refrangibility, those in the violet will be 

 relatively more separated from each other than those in the red. That is to say, if we 

 have two rays, a and b, in the red, whose indices of refraction are almost identical, and 

 which, projected in the spectrum, would stand close to each other, side by side, and 

 another pair, c and d, in the violet, whose indices of refraction bear precisely the same 

 relation to one another, these last, by the act of refraction, will be more separated, be- 

 cause they are relatively more refrangible than the former. The result of this increased 

 separation in the more refrangible regions is to give an apparent dilution to them, while 



