166 Mr. A. A. Common [May 30, 



hinder the light reaching the telescope. In all cases the substance 

 used for the mirrors was what is called speculum metal. 



During the present century the aperture of the refracting 

 telescope has increased enormously ; the manufacture of the glass 

 disks has been brought to a high state of perfection, particularly in 

 France, where more attention is given to this manufacture than in 

 any other country. Early in the century the great difficulty was in 

 making the disk of flint glass. M. Guinand, a Swiss, beginning in 

 1784, succeeded in 1805 in getting disks of glass larger and finer 

 than had been made before, and refractors grew larger and larger as 

 the glass was made. In 1823 we have the Dorpat glass of 9 * 6 inches, 

 the first large equatorial mounted with clock-work ; in 1837 the 

 12-inch Munich glass; in 1839 the 15-inch at Harvard, and in 1847 

 another at Pulkowa; in 1863 Cooke finished the 25-inch refractor 

 which Mr. Newall gave, shortly before his death last year, to the 

 Cambridge University. 



This telescope the University has accepted, and it is about to be 

 removed to the Observatory at Cambridge, where it will be in charge 

 of the Director, Dr. Adams. In accordance with the expressed wish 

 of the late ]VIr. Newall, it will be devoted to a study of stellar and 

 astronomical physics. There is every prospect that this will be 

 properly done, as Mr. Frank Newall, one of the sons of the late Mr. 

 Newall, has ofiered his personal services for five years in carrying on 

 this work. Succeeding this we have the 26-inch telescope at 

 Washington, the 2 6-inch at the University of Virginia, the 30-inch 

 at Pulkowa, and the 3 6 -inch lately erected at Mount Hamilton, 

 California — all these latter by A Ivan Clark and his sons. By Sir 

 Howard Grubb we have many telescopes, including the 28-inch at 

 Vienna. Most of these telescopes have been produced during the 

 last twenty years, as well as quite a host of others of smaller sizes, 

 including nearly a score of telescopes of about 13 inches diameter 

 by various makers, to be employed in the construction of the photo- 

 graphic chart of the heavens, which it has been decided to do by 

 international co-operation. 



The first of these photographic instruments was made by the 

 Brothers Henry, of the Paris Observatory, who have also made 

 many very fine object glasses and specula, and more important than 

 all, have shown that plane mirrors of perfect flatness can be made of 

 almost any size; the success of M. Loewy's new telescope, the 

 equatorial coude, is entirely due to the marvellous perfection of the 

 plane mirrors made by them. 



The reflecting telescope has quite kept pace with its elder 

 brother. 



Lassell in 1820 began the grinding of mirrors, he like Sir 

 William Herschel working through various sizes, finally com- 

 pleting one of 4 feet aperture, which was mounted equatorially 

 Lord Kosse also took up this work in 1840 ; he made two 3-foot 

 specula, and in 1845 finished what yet remains the largest telescope. 



