196 



Lord Rayleigh on Achromatic 

 Fig. 2. 



We have now to inquire under what conditions A/\, or n, 

 will be stationary, in spite of a variation of \, if /3' be con- 

 stant. Thus „ . . , , , 7% 



X sin en! dot + cos u'dX — 0, 



while / 7 , 7 • 7 



cos «'aa = a/ji sm a -f /x cos a aa, 



Accordingly, 



= dfi sin /3 + fACO$ /3 d (3. 



cot a'<A 



so 



that 



cot 2 a!— — 



cos a' = dfju sin a + /x cos a cZa 



= djj, sin a — //, cos a d/3 

 = <^yLt sin a + cos a tan /3 d/j, 

 = sin Adfi/cos /3; 

 X d/j, sin A 



yu, c/X sin a cos /3 



(36) 



is the condition that n should be stationary. In the more 

 particular case considered above, j6' = 0, fi=Q, a=A. 



These bands, which I should have been inclined to desig- 

 nate after Talbot, were it not that his name is already con- 

 nected with another very remarkable system of bands, are 

 readily observed. For the " radiant point of solar light " 

 we may substitute, if more convenient, that of the electric 

 arc. A small hole in a piece of metal held close to the arc 

 allows sufficient light to pass if the bands are observed with- 

 out the intervention of a diffu sing-screen. At a distance of 

 say 20 feet the nearly parallel rays fall upon the prism * and 



* A right-angled isosceles prism (A = 45°) answers very well. The plate 

 should be blackened at the hind surface j or it may be replaced by a 

 second prism. 



