Daguerre lens

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©The third objective invented by Daguerre.

^{par Jacques Roquencourt}

After the objective of the contrat of 1829, the objective of 6.4 inches focal distance (6 pouces), Daguerre will propose the former in 1839.

Daguerre will take a focal distance of 15 inches (14 pouces français) for the format 21.5 cm by 16.5 cm.

Jean-Batiste Biot wrote 18 mai 1839 to Talbot.^***

" it was a matter of curiosity to me to establish the general physical Conditions which Mr Daguerre must , of necessity, have fulfilled in order to obtain the perfection which he has reached in his art.I will tell you that these conditions include purely optical ones which are extremely delicate. I am not talking about the necessity of having a periscopic lens to obtain the focuses on the same plane at various angles of axis. I was well aware that he had observed it. But there is another more delicate requirement. Since each sensitive substance has , so to speak , its own visible spectrum , partly coinciding with the luminous spectrum but also partly outside this spectrum , it follows that curvatures of the lens should be such that it is achromatic for the this ideal spectrum , not for ours ; with the result that it should be appropriate for the special sensitivity of each substance. Now , since I showed my article to Mr Daguerre after I had written it but before it was published , he told me that he had also observed this condition in experiments and that he had varied the Curvatures of his periscopic lens which had been rendered achromatic , until he was satisfied that he had achieved the maximum sharpness. .....he found that the greatest effectiveness was obtained in the blue rays ,.... But you can see what wisdom such remarks show in a man who had never studied optics or specialised chemistry before he came to this Subject........"