by Diffusion of Light. 59 



Lallemand operated also by throwing a pure and polarized 

 spectrum on the coloured diffusing surface : according to the 

 position of the eye, the different diffused colours are perceived 

 to be more or less polarized ; and those which are the least po- 

 larized correspond to the proper colour of the" body. But M. 

 Lallemand attributes this to fluorescence, while, according to 

 the theory which has just been explained, it is the result of 

 absorption and depolarization by interior reflections. 



In support of this theory must be mentioned first the fact 

 that the colour of a body by diffusion is always the same as by 

 transparence. I will here report some experiments which show 

 it in a striking manner : — 



A horizontal pencil of sunlight, polarized in a vertical plane, 

 falls on the coloured diffusing surface. The observer views it 

 under a visual angle of 90°, the plane of vision being horizontal: 

 the colour, as we have said> appears very rich. But as the 

 asperities of the surface are rarely minute enough for the diffusion 

 to be perfect, and in certain cases the substance may be con- 

 tained in a glass vessel, producing specular reflection, it is better 

 to pass the diffused rays through a Nicol or a Foucault, stopping 

 the rest of the polarized rays. The colour of the body then 

 appears with the maximum of saturation and can be analyzed 

 with the spectroscope. 



It is convenient to choose for the operation of diffusion a sub- 

 stance the absorption-spectrum of which is discontinuous and 

 characterized by bands. 



Let us take smalt, for example. Although in powder its 

 colour is much washed with white, because the thickness to be 

 traversed by the light in the interior of the particles is too little 

 to give rise to strong absorption, it is easy to distinguish in the 

 diffusion-spectrum the characters attenuated of the absorption- 

 spectrum of cobalt. 



Let us now take some blood (the spectrum of which is so cha- 

 racteristic) and put it in a glass test-tube, or, still better, moisten 

 with the blood the outside of the tube. Thanks to the Nicol 

 prism placed in front of the spectroscope, the light reflected 



point where the image of the sun is formed, where the pencil is narrowest, 

 a large polarizer is placed. Then, at a distance of one or two metres, in 

 the luminous divergent cone a bouquet of deep-coloured velvety flowers is 

 placed, such as pansies or certain varieties of geraniums. The observer 

 views the bouquet at a right angle with the incident pencil, while an assis- 

 tant turns the polarizer. The tint of the bouquet then appears to pass 

 through alternations of saturated colours and colours washed with white, 

 which are very striking. The observer may also place himself in any posi- 

 tion whatever, if he look through an analyzer which he keeps immovable 

 while the polarizer is rotated, or vice versa. 



