1887.] Lord Bayleigh on the Colours of Thin Plates. 81 



WEEKLY EVENING MEETING, 



Friday, March 25, 1887. 



Sir Frederick Abel, C.B. D.C.L. F.R.S. Manager and Vice-President, 



in the Chair. 



The Eight Hon. Lord Rayleigh, M.A. D.C.L. LL.D. F.R.S. M.B.I. 



On the Colours of Thin Plates. 



The physical theory, as founded by Young and perfected by his 

 successors, shows how to ascertain the composition of the light 

 reflected from a plate of given material and thickness when the 

 incident light is white ; but it does not, and cannot tell us, except 

 very roughly, what the colour of the light of such composition will 

 be. For this purpose we must call to our aid the theory of compound 

 colours, and such investigations as were made by Maxwell upon the 

 chromatic relations of the spectrum colours themselves. Maxwell 

 found that on Newton's chromatic diagram the curve representative 

 of the spectrum takes approximately ^he simple form of two sides of 

 a triangle, of which the angular points represent a definite red, a 

 definite green, and a definite violet. The statement implies that 

 yellow is a compound colour, a mixture of red and green. 



In illustration of this fact, an experiment was shown in which 

 a compound yellow was produced by absorbing agents. An infu- 

 sion of litmus absorbs the yellow and orange rays ; a thin layer of 

 bichromate of potash removes the blue. Under the joint operation of 

 these colouring matters the spectrum is reduced to its red and green 

 elements, as may be proved by prismatic analysis ; but, if the pro- 

 portions are suitably chosen, the colour of the mixed light is yellow 

 or orange. When the slit of the usual arrangement is replaced by a 

 moderately large circular aperture, the prism throws upon the screen 

 two circles of red and green light, which partially overlaj). Where 

 the lights are separated, the red and green aj)pear ; where they are 

 combined, the resultant colour is yellow. 



On the basis of Maxwell's data it is possible to calculate the 

 colours of thin plates and to exhibit the results in the form of a 

 curve upon Newton's diagram. The curve starts at a definite point, 

 corresponding to an infinitely small thickness of the plate. This 

 point is somewhat upon the blue side of white. As the thickness 

 increases the curve passes very close to white, a little upon the green 

 side. It then approaches the side of the triangle, indicating a full 

 orange ; and so on. In this way the colours of the various orders 

 of Newton's scale are exhibited and explained. The principal dis- 



VoL. XII. (No. 81.) G 



