10 Prof. G. Lippmann. On Colour 



TABLE I. Plate with angle of emergence in air 53 24'. 



TABLE II. Plate with angle of emergence in air 54 42'. 



that, as would of course be expected, the most accurate results are 

 obtained with liquids of high refractive index, which give com- 

 paratively large values of a 9. By determining the values of a 

 for each of two optic axes of a given crystal plate, it can easily be 

 ascertained with what amount of accuracy the plate has been cut 

 perpendicularly to the bisectrix. 



The principle of the method here described may very possibly be 

 advantageously employed in other branches of optical investigation. 



" On Colour Photography by the Interferential Method." By 

 G. LIPPMANN, Professor of Physics, Faculty of Sciences, 

 Paris. Communicated by Sir JOSEPH LISTER, Bart., P.R.S. 

 Received April 14, Read April 23, 1896. 



Colour photographs of the spectrum, or of any other object, are 

 obtained by the following method. A transparent photographic film 

 of any kind has to be placed in contact with a metallic mirror during 



