396 Intelligence and Miscellaneous Articles. 



But since, from a law very long ago propounded by M. Becquerel, 

 each metal bears in all the couples the same thermoelectric relation, 

 the electromotive force which has its seat at the junction of the two 

 metals must have the form aF(t) + bF(t). As, moreover, electromo- 

 tive force and electrical tension are synonymous terms, we are led 

 a priori to the conclusion that each body possesses an electrical ten- 

 sion measured by the product of a function of the temperarure iden- 

 tical for all bodies, and of a coefficient special for each of them. 



What is this function of the temperature ? Other experiments will 

 perhaps enable me to determine this. But the identity of this func- 

 tion leads us to conclude that thermo-electricity is a property of mat- 

 ter, and not an accident of a substance. 



As to the conception of an absolute electrical tension, a function of 

 the temperature, I think it is destined to account for many pheno- 

 mena, especially atmospheric electricity, to solve the difficulty of the 

 preexistence of chemical action or of electrical action, and to make, 

 finally, a new step towards the identification of heat with electricity. 

 — Comptes Rendus, August 20, 1866. 



ON A MEANS OF WEAKENING THE SOLAR RAYS IN THE FOCUS OF 

 TELESCOPES. BY M. LEON FOUCAULT. 



When the physical constitution of the sun is to be studied by 

 means of the large instruments in observatories, it is necessary to 

 have recourse to some methods for diminishing the intensity of the 

 light and heat which are concentrated in the focal image. 



By placing a dark glass before the eyepiece, the eye is protected 

 for the first few minutes against the intensity of the radiation ; but 

 if the observation be prolonged, and the object-glass has a large 

 eyepiece, the glass becomes heated and is finally broken, the ob- 

 server being thus exposed to the direct action of the sun's rays. 



Sometimes it has been attempted to remedy this inconvenience by 

 reducing the field of the object-glass by a diaphragm ; but this 

 method only injures the optical effect, and therefore will not bear 

 examination. 



It has further been proposed to make the pencil undergo a partial 

 reflexion under the angle of polarization, and to provide the eye- 

 piece with an analyzer whose azimuth is varied so as to diminish at 

 will the intensity of the rays which traverse it. By this means the 

 images are weakened without any appreciable coloration being im- 

 parted to them ; but it is seldom that they are subjected to this com- 

 plicated treatment without appreciably diminishing the precision of 

 the image : the instrument loses its optical power ; and this is just 

 what must be avoided if we are to obtain from the use of large tele- 

 scopes all they can teach us respecting the revolutions which take 

 place on the star. 



Having been led by my researches on the telescope to silver a large 

 number of glass mirrors, I have often had occasion to remark that 

 the metallic layer whose lustre is so brilliant possesses at the same 

 time a transparence and limpidity comparable to that of the most 

 beautiful coloured glasses. The transparence is so great that, look- 

 ing at the sun through the thin layer of silver, the least vapours 

 passing over its surface are seen distinctly and without the least 

 fatigue. I was naturally led to suppose that a silvered glass might 



