1887.] 



NEW YORK ACADEMY OF SCIENCES. 



105 



(using steam machinery for grinding and polishing) a great 

 mirror, six feet in diameter, the tube being fifty-four feet in 

 length. The telescope has not done as much work as was ex- 

 pected, owing to the bad climate of the region where it is placed. 

 It has been said that there are only a few hours in the year when 

 the telescope can be used with its greatest eflSciency. 



Since 184:2 a considerable number of large reflecting telescopes 

 have been made. But as a general rule (though silvered glass 

 mirrors have been substituted for those of metallic alloy), the 

 reflectors have not given as much satisfaction as was to be ex- 

 pected. The large ones are more difficult to handle ; the mirror 

 tarnishes readily, and has to be frequently resilvered, and the 

 alternations of heat and cold and of flexure produce a distortion 

 of the curve which makes the mirror focus badly. These diffi- 

 culties are so troublesome that refractors are usually preferred. 



The following table gives the location, character and aperture 

 of the great telescopes of the world. 



Size of Principal Telescopes in the World. 



Owner and Location. 

 Lick Observatory, Cal. 

 Pulkova, Russia, 



Refractors. 



Constructed by. 

 A, Clark & Sons, 

 A. Clark & Sons, 



A. [Clark & Sons, 



Grubb, 



A. Clark & Sons, 



Yale College, 



Littrow, Vienna, 



University of Virginia, 



Washington Naval Obser- 

 vatory, A. Clark & Sons, 



Gateshead, England, Cooke, 



Princeton, N. J., A, Clark & Sons, 



Buckingham, London, E'd, Buckingham, 



University of Chicago, A. Clark & Sons, 



Strasbourg, Merz, 



Private Observatory, Buf- 

 falo, Fitz. 



Warner Observatoiy, Ro- 

 chester, Clark Sc Sons, 



Washburne Observatory, 



Madison, Wis., A. Clark & Sons, 



Aperture. 



36 

 30 



28 

 27 

 26 



26 



25 



28 



21 



18.5 



18 



18 " 

 16 " 

 15.50 " 



Remarks. 



in. 

 " Finished ia 



May, 1833. 

 " Constructing. 



