82 Lord Bayleigh on the Colours of Thin Plates. [March 25, 



crepancy between the curve and the descriptions of previous observers 

 relates to the precedence of the reds of the first and second orders. 

 The latter has usually been considered to be the superior, while the 

 diagram supports the claim of the former. The explanation is to be 

 found in the inferior brightness (as distinguished from purity) of 

 the red of the first order and its consequent greater liability to suffer 

 by contamination with white light. Such white light, foreign to the 

 true j)henemenon, is always present when the thin plate is a i)late of 

 air enclosed between glass lenses. To make the comparison fairly, 

 a soaj) film must be used, or recourse may be had to the almost 

 identical series of colours presented by moderately thin plates of 

 doubly refracting crystals when traversed by polarised light. Under 

 these circumstances the red of the first order is seen to be equal or 

 superior to that of the second order. 



[Kayleigh.] 



