Interference-Bands. 193 



Perfect achromatism would be possible only under a law of 

 dispersion* 



fM = A\-* 2 , (26) 



wbere A and c 2 are constants, of which the latter denotes the 

 value of cot 2 a! . 



The above investigation, as given in the Enc. Brit., was 

 intended to apply to Talbot's manner of experimenting, and 

 it affords a satisfactory explanation of the formation of 

 achromatic bands. In order to realize the nearly grazing 

 incidence, the plate of air must be bounded on one side by a 

 prism (fig. 1). Upon this fall nearly parallel rays from a 



Fiff. 1. 



" radiant point of solar light," obtained with the aid of a lens 

 of short focus. The bands may be observed upon a piece of 

 ground glass held behind the prism in the reflected light, or 

 they may be received directly upon an eyepiece. 



These bands undoubtedly correspond to varying thicknesses 

 of the plate of air, just as do the ordinary Newton's rings 

 formed at nearly perpendicular incidence. For theoretical 

 purposes we have the simplest conditions, if we suppose the 

 thickness uniform, and that all the rays incident upon the 

 plate are strictly parallel. Under these suppositions the field 

 is uniform, the brightness for any kind of light depending 

 upon the precise thickness in operation. If the thickness be 

 imagined to increase gradually from zero, we are presented 

 with a certain sequence of colours. When, however, the 

 relation (23) is satisfied, the formation of colour is postponed, 

 and the series commences with a number of alternations of 

 black and white. In actual experiment it would be difficult 

 to realize these conditions. If the surfaces bounding the 

 plate are inclined to one another, the various parts of the 

 field correspond to different thicknesses ; and, at any rate 

 if the inclination be small, there is presented at one view a 

 series of colours, constituting bands, the same as could only 



* A mistake is here corrected. 



