Constitution of Natural Radiation. 575 



does not, however, seem to be in what follows anything that 

 stands in definite contradiction with Lord Rayleigh's published 

 views. 



That there is room for still further precision of terminology 

 in this subject is indeed suggested by the beginning of 

 Lord Rayleigh's paper. It would seem that a definite 

 choice can be made between the two modes of exposition, 

 both of which he considered to be allowable. According to 

 the first, " The assertion that Newton's experiments prove 

 the colours to be already existent in white light is usually 

 made in too unqualified a form." On a first impression this 

 remark might be imagined to strike at the roots of all the 

 various instrumental methods that have been elaborated for 

 analysing complex radiations ; for if the analysis brings out 

 features that are not already existent in the radiation, two 

 different methods of analysis (e. g., by a grating and by a 

 prism) can hardly be expected to give concordant results. 



The alternative mode of exposition is to say that each 

 complex type of radiation is constituted definitely of those 

 colours (simple trains of various wave lengths), into which 

 the Fourier mathematical analysis would divide its vibration- 

 curve : and that various analysing instruments (gratings, 

 prisms, &c.) are capable of revealing this constitution with . 

 different amounts of precision, the outstanding differences 

 between these analyses being treated as due to imperfections 

 of the instruments as regards the purpose in question. This 

 (the usual) point of view is claimed as a valid alternative in 

 Lord Rayleigh's third paragraph : to hold, as is done infra, 

 that in some cases the resulting analysis is so imperfect as to 

 be valueless, need not disturb the general validity of this 

 point of view. 



So far, the matter is one as to the most suitable mode of 

 theoretical description or formulation. But we presently 

 reach questions on v\ hich opinions may perhaps differ as to 

 physical fact. In the spectral analysis of ordinary con- 

 tinuous radiation, the prism and the grating give consistent 

 results, when well understood corrections and adaptations are 

 applied before making the comparison. If these instruments 

 are applied to radiation consisting of a system of sharp, 

 entirely uncoordinated, discrete pulses, such as the Rontgen 

 rays are supposed to be, will this general agreement continue ? 

 It is clear that the grating (if of ideal perfectly reflecting 

 quality) will draw out each pulse into a spectrum, and thus 

 will analyse the radiation. It seems open to question whether 

 a prism will not merely gradually dissipate it by scattering, 

 however wide the pulses may be, even if they are of breadth 

 comparable with the wave-lengths of visible light. If this 



