Aplanatic Object-glasses, 258 



c urves, thicknesses, and diameter, &c. of that marked 14,- which 

 I here give (unfortunately one of the least effective of the set,) 

 however, in all probability, the rest are constructed on the same 

 principle ; the annexed drawing by Mr. L. will sufficiently 

 explain itself. Nothing can surpass the beautiful simplicity of 



Chevalier's, or rather Euler's, curves, which, it will be ob- 

 served, are all alike*. The production of cZeep achroma tics 

 must ever be a task of some difficulty, even to those who 

 thoroughly understand their humours and punctilios ; and un- 

 scientific artists will, I think, be much more likely to succeed 

 on the French plan, than the English one. Two double object- 

 glasses, by themselves, are very poor things ; but, when com- 

 bined, perform admirably, and will, I believe, (if the three 

 curves are of equal radii,) be far more easily executed than 

 one of the triple TuUeian construction. The dense flint-glass 

 of Guenard, or Frauenhofer, however, will be an indispensable 

 requisite, from the nature of the curvatures. A triple Tulleian 

 object-glass, and a thin double one of Chevalier, (a composition 

 first conceived and adopted by Mr. Lister,) form an excellent 

 combination, and give a very vivid light, without softness, 

 dulness, or nebulosity. This, I think, is the extreme number 

 of glasses which ought to be tolerated. Let it never be for- 

 gotten, that a really good triple glass will bear an aperture 

 quite sufficient far ninety-nine objects out of a hundred. I 

 myself denounce the practice of combining glasses together, 

 in all those cases where they are capable of doing their work 

 alone, I shall always consider it as a clumsy, bungling, and 

 unworkmanlike method of obtaining a short focus, combined 



♦- The theory on which these object-fi;lasses are constructed is con- 

 tained in a paper of Euler's, published at St. Petersburgh, in 1 774. 

 Messrs. Chevalier have caused it to be inserted entire in the '* Bulletin 

 de la Societe d' Encouragement de Paris'," No. C!CLIV., for Aug. 1825. 



OCT.— DEC 1827, S 



