224 ANNUAL OF SCIENTIFIC DISCOVERY. 



curves, the centerage and the execution of four spherical surfaces. In the 

 new telescope, on the contrary, the glass, serving not as a middle refractor, 

 but only to support a very thin layer of metal, the homogeneity of the mass 

 is by no means required, and the most ordinary glass of sufficient thickness 

 worked with care, affords a concave surface, which when silvered and pol- 

 ished furnishes of itself and by reflexion excellent images. There is one 

 strong objection to the metal mirrors, it is, that they become oxidized in 

 time, and are tarnished by contact with the air. Eight months I have sil- 

 vered mirrors, which have not yet undergone any sensible alteration. Will 

 they preserve this state of perfection a still longer time 7 The experiment 

 has not yet been sufficiently prolonged to decide one way or the other ; but 

 even should the lustre of the mirror become weaker, there is no difficulty in 

 recurring to the same means for re-establishing it, by \vhich it had been at 

 first obtained. In fine, should the depth of the silver be altered, the opera- 

 tion of depositing it is so easy and prompt, that it can easily be repeated. 

 To resume, the new instrument compared with the refracting telescopes 

 gives, at much less cost, more light, more distinctness, and is free, like the 

 reflecting telescope, from all aberration of refrangibility. 



Professor Stoney asked what the material of the polisher used by M. 

 Foucault was, and gave various reasons for doubting that the method pro- 

 posed by the author would ever produce a speculum the defining power of 

 which could approach to that of specula ground and polished by the methods 

 devised and executed by Lord Rosse. Mr. Grubb stated, that if the one 

 three thousandth of an inch spoken of by M. Foucault was meant to convey 

 an idea that that dimension bore any relation to the quantity removed in the 

 process of polishing, his own experience would enable him confidently to 

 deny its power of producing a speculum of accurate defining power, as in 

 the polishing process thicknesses of a forty and fifty thousandth part of an 

 inch became important thicknesses. The President, Dr. Robinson, said, 

 that when M. Foucault had visited Lord Rosse, as he was about to do, and 

 had seen the apparatus which he used for grinding and polishing even mon- 

 ster specula, he would not, he felt well assured, consider these operations on 

 metals so formidable as they now appeared to him ; he would find that to 

 polish the great speculum of six feet diameter, in which operation it was 

 brought to the true figure for best definition, occupied only a matter of 

 about five hours from the time it was placed upon the machine. 



ON THE CENTERING THE LENSES OF COMPOUND OBJECT-GLASSES 



OF MlCIiOSCOrES. 



The following is a report of a communication made by Sir David Brew- 

 ster to the British Association at its last meeting at Dublin, on the above 

 sul iject. 



In studying the subject of diffraction, as seen through the microscope, I 

 was led to believe that in the best object-glasses now made, the axes of the 

 individual lenses were now coincident. I have no means or learning by 

 what process the optician centres his lenses, and groups of lenses, but it 

 must be a verj delicate one, when we consider the small size of the lenses 



