NATURAL PHILOSOPHY. 141 



two ends of the wire, and to the conjectures of impediments to conduc- 

 tion, and of accumulation and discharges in a cm-rent ; as that which 

 was once regarded as intermediate conduction in the earth is now 

 supposed to belong only to an equalization or to a restoration of elec- 

 tric tension. Although, according to the present limits of exactness 

 in this kind of observation, it is probable the aberration is constant, 

 and, therefore, the velocity of light, of all the fixed stars, is the same, 

 yet the possibility has more than once been spoken of that there may 

 be luminous bodies in space whose light does not reach us because, 

 from their enormous mass, gravitation constrains the luminous par- 

 ticles to return. Humboldt's Cosmos. 



CONSTRUCTION OF MIRRORS FOR REFLECTING TELESCOPES. 



LORD ROSSE, at the British Association, after adverting to the difficul- 

 ties in the way of perfecting the reflecting telescope, stated that he had 

 come to the conclusion that the only recourse in guarding against them, 

 consisted in improving the metallic plane reflectors as much as possible. 

 As a material for reflectors he had tried silver ; but, unfortunately, this 

 metal was so soft that great difficulties presented themselves in giving 

 it the requisite degree of high polish. He had tried, by the electro- 

 type process, to procure a surface with a high polish, by depositing 

 silver on a surface of speculum metal ; but, unfortunately, after every 

 precaution, the silver adhered to the metal. He tried copper similarly, 

 which did not adhere, but produced a high degree of polish ; its color, 

 however, and other properties, rendered it inadmissible as a reflector. 

 He then determined to endeavor to grind and polish a plane surface of 

 silver, the softness of that metal having, ho\vever, heretofore caused 

 the attempt to fail in the hands of the most experienced who had tried 

 it. The processes of grinding and polishing are essentially different. 

 In grinding, the substance, whether emery or other powder, must run 

 loose between the substance which is used to rub it against the other 

 and that which is to be ground ; and he soon found that he could not 

 use emery or any other grinding powder for bringing a surface of silver 

 to a correct form ; for, from the softness of the metal and the unequal 

 hardness of its parts, the emery was found to confine its action to the 

 softer parts, leaving the harder portions in elevated ridges and promi- 

 nences, something in the way that the iron handle of a pump which 

 has been long and much used may be observed to be worn away. 

 Hard steel he found he could bring to a very true surface, and even 

 impart to it a high degree of polish ; but the quantity of light it was 

 capable of reflecting was by no means sufficient ; nor could he succeed 

 in imparting to the surface of silver by compression with highly pol- 

 ished steel surfaces the evenly and highly polished surface requisite for 

 his purpose. At length, he found that he could, by the use of good 

 German hones, grind surfaces of silver perfectly true ; and he had now 

 no doubt that he could with safety recommend for that purpose, as the 

 best material, the blue variety of German hone. The next point was, 

 to polish the surface to a true optical plane reflecting surface. This 



