ADAPTED FOR OPTICAL PURPOSES. 27 



arising, Mr. P. Barlow has been led to turn his attention to the 

 substitution of fluids for flint-glass, in the construction of large 

 object-glasses; and he has had great success in the use of this 

 most ingenious expedient; though it is probably one which 

 would be immediately abandoned were perfect glass to be 

 obtained. On the same account, the recent acquisition of a 

 lens of flint-glass, of eleven inches in diameter, by the Presi- 

 dent of the Astronomical Society, (Sir James South,) has 

 created a sensation in the world of science, which those who 

 were unacquainted with the enormous difficulty of obtaining a 

 lens of that magnitude, fit for use in a telescope, would be 

 altogether unprepared to appreciate. 



These difficulties have induced some persons to labour hard 

 and earnestly for years together, in the hope of surmounting 

 them. Among them were ingenious manufacturers and work- 

 men, and profound men of science. But the former, however 

 skilful in the practical details of glass-making, were unac- 

 quainted with the true causes of the imperfections they were 

 desirous of removing ; and it happened that the attention of 

 the latter was directed rather to the obtaining of a stock of 

 perfect glass for present and individual use, than to such an 

 improvement of the manufacture, as would place the acquisi- 

 tion of such glass within the means of all who were qualified 

 to apply it. The result was, that although some perfect glass 

 in large pieces was produced by the late M. Guinand, an 

 ingenious artist of Bale, and by the late M. Fraunhofer of 

 Munich, an optician, and an original discoverer in optics, of 

 the highest merit; yet the knowledge of the means of producing 

 perfect glass which they acquired, seems to have been altogether 

 practical and personal, a matter of minute experience, and not 

 of a nature to be communicated to others. It is certain, at 

 all events, that down to the end of the year 1 829, the public 

 were not in possession of any instructions relative to the me- 

 thod of making homogeneous glass fit for optical purposes, be- 

 yond what had been possessed before either M. Fraunhofer, or 

 his ingenious contemporary, had commenced their respective 

 operations; and it seemed doubtful, in this country, (to use the 

 words of a recent writer on the subject, when describing his 

 own researches upon it, to which I shall immediately advert,) 

 whether the meritorious foreigners I have named " ever 

 attained a method of making such glass with certainty and at 

 pleasure, or have left any satisfactory instructions on the sub- 

 ject behind them." 



Such, within these few years, were the exigencies of science 

 with respect to the manufacture of glass for optical purposes, 

 and such was the progress which had been made in endea- 



