78 Lord Rayleigh on Achromatic 



but the system is far from achromatic. Theoretically there is 

 not even a single place of darkness, for there is no point where 

 there is complete discordance of phase for all kinds of light. 

 In consequence, however, of the fact that the range of sensi- 

 tiveness of the eye is limited to less than an " octave/' the 

 centre of the first dark band on either side is sensibly black ; 

 but the existence of even one band is due to selection, and the 

 formation of several visible bands is favoured by the capa- 

 bility of the retina to make chromatic distinctions within the 

 range of vision. After two or three alternations the bands 

 become highly coloured*; and, as the overlapping of the 

 various elementary systems increases, the colours fade away, 

 and the field of view assumes a uniform appearance. 



There are, however, cases where it is possible to have 

 systems of achromatic bands. For this purpose it is neces- 

 sary, not merely that the maxima of illumination should 

 coincide at some one place, but also that the widths of the 

 bands should be the same for the various colours. The inde- 

 pendence of colour, as we shall see, may be absolute ; but it 

 will probably be more convenient not to limit the use of the 

 term so closely. The focal length of the ordinary achromatic 

 object-glass is not entirely independent of colour. A similar 

 use of the term would justify us in calling a system of bands 

 achromatic, when the width of the elementary systems is a 

 maximum or a minimum, for some ray very near the middle 

 of the spectrum, or, which comes to the same, has equal values 

 for two rays of finitely different refrangibility. The out- 

 standing deviation from complete achromatism, according to 

 the same analogy, may be called the secondary colour. 



The existence of achromatic systems was known to Newton f, 

 and was insisted upon with special emphasis by Fox Talbot J; 

 but singularly little attention appears to have been bestowed 

 upon the subject in recent times. In the article " Wave 

 Theory " (Encyc. Brit, ] 888) I have discussed a few T cases, 

 but with too great brevity. It may be of interest to resume 

 the consideration of these remarkable phenomena, and to 

 detail some observations which I have made, in part since the 

 publication of the ' Encyclopaedia ' article. A recent paper 

 by M. Mascart § will also be referred to. 



* The series of colours thus arising are calculated, and exhibited in the 

 form of a curve upon the colour diagram, in a paper " On the Colours of 

 Thin Plates/' Edinb. Trans. 1887. 



t ' Optics,' Book ii. 



X Phil. Mag. [3] ix. p. 401 (1836). 



§ " On the Achromatism of Interference," Comptes Rendus, March 

 1889 ; Phil. Mag. [5] xxvii. p. 519. 



