128 PROGRESS Iisr ASTRONOMY. 



The long supremacy of Germany in the matter of refractors was 

 broken down iiltimatel}^ by the famous English optician and engineer, 

 Thomas Cooke, of York. His first considerable instrument, one of 7 

 inches aperture, was finished in 1S51, and in 1865, a year before his 

 lamented death, he completed the first of our present giant refractors, 

 one of 25 inches aperture, for Mr. Newell, of Gateshead. In conse- 

 quence of the success of Cooke's achievement other large refractors 

 were soon undertaken. 



Alvan Clark, the famous optician of Cambridgeport, Mass., at once 

 commenced a 26-inch for the Washington Observatory. The next was 

 one of 27 inches, made by Grubb for the Vienna Observatoiy. Object 

 glasses now grew inch by inch in size, depending on the increased 

 dimensions of disks that could be satisfactorily cast. Gautier of Paris 

 completed a 29i-inch for the Nice Observatory, while Alvan Clark 

 made an objective of 30 inches for Pulkowa. In 1877 the latter suc- 

 cessfully completed the mounting of an objective of 36 inches for the 

 Lick Observatory, but this inmiense lens was only achieved after a 

 great number of failures. Even this large object glass was surpassed, 

 in size by the completion in 1892 of the -lO-inch, which he made for the 

 Yerkes Observatory, and b}^ that made by Gautier for the Paris 

 Exhibition of 1900. 



So much, then, for the largest refractors. In recent years, since the 

 introduction of the silver on glass mirrors, with their stability of 

 figure and brilliant surface, which can be easily renewed, reflectors of 

 large apertures are again being produced. The first of these was one 

 of 36 inches aperture made by Calver for Dr. Common, who demon- 

 strated its fine qualities and his own skill by the beautiful photographs 

 of the nebula of Orion he was enabled to secure with it. Dr. Com- 

 mon himself has since turned his attention to the making and silvering 

 of large mirrors of this kind, and the largest he has actually completed 

 and mounted eqtiatorially is one with a diameter of 5 feet. Another 

 of 36 inches aperture is in use at the Solar Physics Observatory, at 

 Kensington. 



The progress of depositing silver on glass has led of late years to 

 important developments in which plane mirrors are used. Foucault 

 was the first to utilize such mirrors in his siderostat, in which such 

 a mirror is made to move in front of a horizontal fixed telescope, which 

 may be of any focal length, and no expensive dome or rising floor is 

 required. The plane mirror of the siderostat in the Paris Exhibition 

 telescope is 6 feet in diameter. 



A variation of this instrument is the coelostat, more recently advo- 

 cated by Lippmann. The Coude equatorial mounting also depends 

 upon the use of plane mirrors; with such a telescope the observer is 

 at rest at a fixed eyepiece or camera in a room which may be kept at 

 any temperature. 



