232 On the Manufacture of Optical Glass. [1829. 



found liable to certain imperfections, not essentially existing, 

 but almost always involved during its preparation, and fatal to 

 its use. These are so important and so difficult to avoid, that 

 science is frequently stopped in her progress by them ; a fact 

 fully proved by the circumstance that Mr. Dollond, one of our 

 first opticians, has not been able to obtain a disc of flint-glass 

 four inches and a half in diameter, fit for a telescope, within 

 the last five years, or a similar disc of five inches in diameter 

 within the last ten years. 



It must be well known to the scientific world, that these 

 difficulties have induced some persons to labour hard and 

 earnestly for years together, in hopes of surmounting them. 

 Guinand was one of these : his means were small, but he de- 

 serves the more honour for his perseverance and his success. 

 He commenced the investigation about the year 1784, and died 

 engaged in it in the year 1823. Fraunhofer laboured hard at 

 the solution of the same practical problem. He was a man of 

 profound science, and had all the advantages arising from 

 extensive means and information, both in himself and others. 

 He laboured in the glass house, the workshop, and the study, 

 pursuing without deviation the great object he had in view, 

 until science was deprived of him also by death. Both these 

 men, according to the best evidence we can obtain, have pro- 

 duced and left some perfect glass in large pieces : but whether 

 it is that the knowledge they acquired was altogether practical 

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

 to be communicated ; or whether other circumstances were 

 connected with it, it is certain that the public are not in pos- 

 session of any instruction, relative to the method of making a 

 homogeneous glass fit for optical purposes, beyond what was 

 possessed before their time ; and in this country it seems doubt- 

 ful whether they ever attained a method of making such glass 

 with certainty and at pleasure, or have left any satisfactory in- 

 structions on the subject behind them. 



The philosophical deficiencies referred to above, induced the 

 President and Council of the Royal Society in 1824, to appoint 

 a Committee for the improvement of glass for optical purposes, 

 consisting of Fellows of the Royal Society and Members of the 

 then Board of Longitude. The Government on being applied 

 to, not only removed the restrictions to experiments on glass, 



