42 Oil Coloured Shadows. 



" 1. Coloured shadows may be distinguished into objective 

 and subjective. 



" 2. The former owe their colouring to the light which ar- 

 rives at them either directly or by reflection ; they are not 

 therefore total shadows, but are rather, to use the scientific 

 term, penumbrcB. 



" 3. The shadows whose colouring is subjective, are the ef- 

 fect of a particular disposition of our" organ, which, when it is 

 fatigued by the impression of a single colour, no longer per- 

 ceives that ray in a fasciculus of white light ; so that the com- 

 plementary ray predominates and communicates its tint to the 

 shadow projected in the primitive light. 



" 4. So far as we have been able to observe, the eye follows 

 in this process the scale of Newton. If the corresponding co- 

 lours are not always exactly coniplementary, it must be attri- 

 buted to the difficulty of obtaining artificial tints so pure as those 

 of the solar spectrum. 



" 5. There follows from this, that the colouring of shadows 

 is impossible, if there be no other light than that by the inter- 

 ception of which the shadow is formed. The presence of a light 

 coming from another part, for example, from the sky, or the 

 clouds, is an indispensable condition to the formation of coloured 

 shadows. 



" 6. Lastly, the shadow is not necessary to make the comple- 

 mentary colours appear. A small quantity of white light, put 

 in prominent contrast with a large mass of coloured light, as- 

 sumes, in certain circumstances which we cannot well determine, 

 the complementary tint corresponding to the colour of this latter 



light." 



Mr TrechsePs explanation appears to us satisfactory ; it intro- 

 duces, it is true, two causes for a phenomenon which has usually 

 been considered as one; but this is not the first case where 

 a careful analysis has obliged us to admit several agents in an 

 effect single in appearance. Without doubt, in the number of 

 the very varied experiments which may be made on the subject 

 of coloured shadows, there will still present themselves many de- 

 tails which will not be immediately explained ; but it is probable 

 that their origin will be found in the peculiar circumstances of 



