582 



The solar action causes the three simple colours to be developed 

 successively, and in the following order, yellow, blue, and red. 

 Between these, the compound colours green and violet which result 

 from their mixture, are obtained with the greatest distinctness if the 

 action is slow. But whilst the yellow disappears by prolonged 

 action, a considerable amount of blue always remains ; whence in 

 nature the final red is never pure, so that the dye always inclines 

 more or less to violet. 



These properties have been placed beyond doubt by the possibility 

 of making photographs on silk and cambric, which exhibit a remark- 

 able delicacy in detail, combined with great strength of tone. 



In a photograph obtained in this way, the different tints through 

 which the dye-stuff passes before becoming violet are more or less to 

 be seen, but the deep violet predominates, and represents the black 

 of ordinary photographs. 



The changes in the colour of the purple dye-stuff are accompanied 

 by the production of a very penetrating foetid odour, similar to that 

 of essence of garlic. The evolution of this odour is as charac- 

 teristic of the solar action as the changes of colour, a consideration 

 of much importance when we desire to solve the problem to which 

 I now turn What was the primitive colour of the purple stuffs of 

 antiquity ? 



At first sight this question seems to be easily answered; but 

 when one seeks for a precise signification of the word " purple," one 

 soon becomes embarrassed. If we ask a painter, without telling him 

 why, Be so good as to paint the shade which you would give to 

 a purple drapery in a historical painting each painter to whom 

 the request is made will give a different colour. This is the case 

 because no one has an exact idea of the primitive colour, which has 

 been gradually modified, and which has now become the red, almost 

 scarlet, which many painters understand by the word purple. It 

 is only by the interpretation of the phrases of the ancients, and com- 

 paring them with direct observations, that one arrives at a solution 

 of the difficulty, which would appear to be of great use to art. 



It is enough to remark that the purple colour exists only because 

 it has been developed by the sun, in justification of the conclusion 

 that the ancients must have been acquainted with this peculiarity, 

 as also with that of the development of the characteristic foetid 



