336 Mr Blair on Achromatic Telescopes 



ture, the time has now come for retrieving our character, and 

 replacing us in the position from which we have been driven. 



Professor Barlow informs us that he has been very greatly 

 indebted for his success to the liberal scientific views and prac- 

 tical ingenuity of Messrs W, and T. Gilbert, mathematical in- 

 strument makers to the Honourable East India Company, and 

 we have no doubt, from our own knowledge of their ingenuity 

 and enterprise, that these able aa:tists will now take a more pro- 

 minent place in their profession than they have hitherto done. 

 They are now occupied in constructing for the Commissioners 

 of the Scottish Light-houses one of the polyzonal or built up 

 lenses, invented by Dr Brewster. This lens is to be made of 

 flint glass, and is to have a diameter of no less than three feet. 

 One of the zones has been for some time finished, and we ex- 

 pect that it will be completed in such a manner as to ad- 

 vance the reputation of the artists, and do honour to the arts of 

 Great Britain. 



Art. XXVI. — On the Permanency of Achromatic Tele- 

 scopes constructed with Fluid Object-Glasses. By Archi- 

 BALD Blair, Esq. In a letter to Dr Brewster. 



Dear Sir, 



As I have been for some time occupied with the construction 

 of aplanatic telescopes on the principle discovered by my fa- 

 ther, in which the aberrations are destroyed by means of a 

 fluid which disperses the variously coloured rays of the spec- 

 trum in the same proportion as crown glass, with a view to 

 the establishment of a regular manufacture of these instru- 

 ments, it will not be thought improper to give an account of 

 the experience which has been already had of the permanency 

 and general practical utility of instruments constructed on 

 this principle, as it has been believed that there are certain 

 practical objections which have hitherto prevented them from 

 being brought into general use. 



In the first place, it has been very generally doubted whe- 

 ther any method had been discovered of enclosing the fluid 

 so perfectly as to prevent tiie risk of its escape at a future pe- 



