Intelligence and Miscellaneous Articles. 397 



replace coloured glasses, and that it would have over these latter the 

 great advantage of reflecting all rays which do not pass through it. 

 Most certainly a glass plate silvered on one face and placed in the 

 body of the telescope in the path of the beam, would present a con- 

 venient means of viewing the sun. 



But as this layer of silver might be regarded as a medium with- 

 out thickness, I thought it would be better to silver the object-glass 

 itself, and to leave quite untouched the arrangement of the telescope. 



I did not alter the eyepieces ; I left the micrometer in its place, 

 and confined myself to silvering the outside of the object-glass. By 

 this means the instrument is protected against the ardour of the sun's 

 rays, which are almost entirely reflected towards the heavens, while 

 the small portion of bluish light which traverses the layer of metal is 

 refracted in the ordinary manner, and forms in the focus a pure and 

 calm image which may be observed without danger to the sight. 



The contour of the disk stands out distinctly on a dark sky, the 

 spots are well defined, the faculae are distinctly seen as well as 

 the decrease of light towards the edges, and the observer imme- 

 diately feels that he is provided with a powerful means of investi- 

 gation. The true tint of the sun is a little altered by the predomi- 

 nance of the blue rays ; but the ratios of intensity are so well pre- 

 served that no detail is lost, and after a certain time the eye, accus- 

 tomed to this bluish colour, ceases to be conscious of it. 



It is true that a telescope thus prepared is an instrument sacri- 

 ficed, at all events for a time, to a single object. Perhaps it will be 

 found that the object is worth the sacrifice. At a time when the greatest 

 questions are discussed regarding the physical constitution of the sun, 

 when the newest and most ingenious lights tend to unveil the me- 

 chanism of such a prodigious effusion of heat and light, it will doubt- 

 less not be uninteresting to try an application on a large instrument. 



In reply to a question put by M. Chevreul as to whether in the 

 case of gold-leaf blue light is transmitted through the substance of 

 the gold, or simply passes through the numerous interstices of a 

 plate reduced by mechanical means to so slight a thickness, M. 

 Foucault said that in his opinion the blue coloration of light trans- 

 mitted through gold-leaf proves that this metal, like silver, can allow 

 light to pass by a real transparence, but that this does not prevent 

 the gold in this extreme degree of fineness from affording numerous 

 chinks invisible to the microscope, and which allow a certain quantity 

 of direct light to pass. Something of the same kind is met with in 

 the case of silver deposited under certain conditions, although the 

 microscope does not furnish a proof. When the reagents are put 

 together the proportions may be altered, and the layer ceases to pre- 

 sent its characteristic blue tint. Everything leads to the belief that 

 the layer of metal thus formed has not a perfect continuity ; for by 

 rubbing the metallic surface with a skin its metallic lustre is increased, 

 the quantity of transmitted light diminished, and the blue colour 

 made to reappear. Evidently under the pressure of the burnisher 

 the silver is extended, the pores are closed, and the light can now 

 only pass through the silver itself. What is here proved for gold 

 and for silver would doubtless apply to all metals if they could be 

 reduced into sufficiently fine plates. — Comptss Rendus, Sept. 3, 1866. 



