96 JAMES CLERK MAXWELL 



beams which differ optically may yet bo alike chroma- 

 tically ; it is possible by mixing red and yellow to 

 obtain an orange colour chromatically similar to the 

 orange of the spectrum, but optic-ally different to that 

 orange, for the compound orange can be analysed by 

 a prism into its' component red and yellow; tho 

 spectrum orange is incapable of further resolution. 

 Newton also solves the following problem : 

 In ci niixttti* of jn'iitt<iri/ <Wo<// % y, tltt* qiutntity 

 and quality of each bciity yiren to know //" rulour 

 of the compound (Optics, Book 1, Part 2, IVop. (I), 

 and his solution is the following: lie arranges tho 

 seven colours of the spectrum round the circumfer- 

 ence of a circle, the length occupied by each colour 

 being proportional to the musical interval to which, 

 in Newton's views, tho colour corresponded. At the 

 centre of gravity of each of theso arcs he supposes a 

 weight placed proportional to the number of rays of 

 the corresponding colour which enter into the mixture 

 under consideration. The position of the centre of 

 gravity of these weights indicates the nature of the 

 resultant colour. A radius drawn through this centre 

 of gravity points out the colour of the spectrum which 

 it most resembles; the distance of the centre of gravity 

 from the centre gives the fulness of the colour. 

 The centre itself is white. Newton gives no proof 

 of this rule ; he merely says, " This rule I conceive to 

 be accurate enough for practice, though not mathe- 

 matically accurate." 



Maxwell proved that Newton's method of finding 

 the centre of gravity of the component colours was 

 continued by his observations, and that it involves 



