84 SECTION PHYSICS. 



must be named on account of the polarizing microscopes which he uses 

 for the investigation of crystals, and which complete, in another branch, 

 the series of goniometers of Charles, and Babinet, and Senarmont. 



The spectroscope is now largely employed for industrial purposes. 

 Here is the whole collection of instruments of M. Dubosq and 

 Laurent, who have with such signal results followed in the steps 

 of M. Soleil. 



You know M. E. Becquerel's phosphoroscope, but I advise you to ex- 

 amine the small tablet upon which one of our most skilful glass-blowers 

 has written in large letters this one word : Phosphorescence. If this little 

 plate be exposed for a few instants to the rays of the sun, it is affected 

 in such a way that all the letters appear luminous and assume different 

 colours. The phosphates, with which the tubes are filled, then vibrate, 

 each in its own fashion, conformably to the principles laid down by M. 

 Edmond Becquerel, and the light which it gives out is of such brilliancy 

 that it makes this phenomenon of phosphorescence one of the most 

 beautiful of optical experiments. 



I will point out presently the heliostats of s'Gravcsande, of Silbcr- 

 mann, of Gambey, and of Foucault, which are in this exhibition. They 

 form an almost complete collection of these instruments so precious for 

 observations. 



We have decided to entrust to you Daguerre's second proof. It 

 is with great satisfaction that we have noticed the precautions 

 you have taken by placing it in a red glass frame to preserve 

 in a fitting manner this first specimen of a great art. And we are most 

 happy to have been able to place by the side of the historical picture of 

 the Photographic Society of France, in addition to Daguerre's attempts, 

 those lent to us by M. Fizeau, and which represent the fixture by 

 chlorate of gold, and his process of photo-engraving which marks a 

 first and most important epoch in the annals of photographic repro- 

 ductions. The stone and the proof by Poitevin call to mind undoubtedly 

 the most important step which has been taken since then. 



Daguerre's attempt carried out in red glass, without mercury, by 

 M. E. Becquerel, and especially his coloured spectrum, are also very 

 striking curiosities of photography. M. A. Girard's microscope is a 

 more practical object : it gives directly magnified impressions of 

 microscopic objects placed before the object glass. 



