436 HISTORY OF SCIENCE. 



tion of the silvered glass specula has stimulated the makers of achro- 

 matic lenses to, give them increased dimensions. The difficulties 

 connected with the construction of lenses of larger diameter are very 

 great. It is no easy matter to obtain glass discs of considerable size 

 perfectly uniform throughout in their refracting power, and free from 

 stria and other defects. At the end of the last century a Swiss me- 

 chanic named Guinand succeeded, after long-continued attempts, in 

 producing flint-glass suitable for the construction of achromatic object- 

 glasses. The manufacture of flint-glass had, before this, been carried 

 on in England only, and this country consequently enjoyed almost a 

 monopoly in the making of achromatic lenses. But when intelligence 

 of the success of the Swiss artizan reached Munich, FRAUNHOFER 

 (1787 1826), the famous optical instrument maker, induced Guinand 

 to remove to Bavaria about the year 1805. Fraunhofer then began 

 to construct achromatic telescopes of diameters far exceeding anything 

 hitherto attempted. With a telescope of Fraunhofer's construction 

 of 10 inches aperture, Struve made his famous observations on double 

 stars between 1824 and 1837; and, with another achromatic of 12 

 inches, Lament of Munich made a series of interesting observations 

 of the satellites of Uranus. Guinand returned to his native country 

 in 1814, and subsequently supplied some of his now celebrated flint- 

 glass to certain eminent French opticians, who most skilfully used it 

 in the construction of achromatic lenses. Achromatic object-glasses 

 of 2 feet diameter and upwards have been since constructed by Cooke, 

 Clarke, Grubb, and other optical artists. 



Stellar parallax was the subject of several investigations in the earlier 

 part of the present century, but it was only about 1843 that the parallax 

 of certain fixed stars was established and its amount ascertained. There 

 is a star registered by astronomers as 6 1 Cygni, which is remarkable 

 for the relatively large amount of \\s proper motion. That is, the posi- 

 tion of the star in the heavens is changing so that it annually displaced 

 5" of arc, or in about 360 years it will occupy a position separated 

 from its present by an interval equal to the breadth of the sun or 

 moon. Bessel concluded that as the movement of the star is greater 

 than that observed in other stars, probably 61 Cygni was compara- 

 tively nearer to us. The star is one of the doubles having two 

 components, and the angular distances of these from two very small 

 stars near them was carefully determined at different seasons of the 

 year. After all allowances and corrections had been made, Bessel's 

 final result was that the annual parallax of 61 Cygni amounts to 0-3483". 

 A subsequent and independent investigation by M. Peters at the Pul- 

 kowa Observatory gave 0-349" as the parallax of the same star. It 

 will be noticed that the correspondence of the results is very close. 

 Another star, having a large proper motion, a Centauri, was observed 

 for parallax by Henderson at the Cape of Good Hope in 1832-3, and 

 the result deduced was i'i6". Maclean, who succeeded Henderson 



