228 M. Chevalier's Answer to Dr Goring' s 



ploy the object-glasses 1, 2, 3, to obtain the maximum. (See 

 Remark F.) 



The examination which you will be pleased to make, Sir, 

 oF the last sextuple object-glass which I have addressed to you 

 composed of 



1 lens meniscus, focal distance 4 lines. 

 1 do. - 3 



1 do. - - 2 



will confirm, I hope, the very correct opinion you have that the 

 perfection of object-glasses is found in short Joci. (See Remark 

 G.) Therefore, it is towards this point that our exertions will 

 be directed, and I think that in a few months we shall in- 

 crease the power of our last set of object-glasses, (see Remark 

 H) composed of 



1 lens plano-convex, focal distance 4 lines. 



1 meniscus 2 do. 



1 do. - - 2. do. 



I think that opticians must direct their attention and their 

 care to achromatic lenses, of a large aperture, thin, of small 

 diameter, and of the shortest possible focal distance, and when 

 thev have succeeded in perfecting them, then will microscopes 

 be carried to their highest degree of excellence. 



I have read the interesting paper of Mr Lister upon a new 

 method of correcting the object-glasses of microscopes. I think 

 that this plan will give facilities for the fabrication of object- 

 glasses of a moderate focus, and that by this discovery, Mr 

 Lister has advanced a step farther towards the perfection of 

 microscopes. 



Professor Airy proposes in his papers to supplant the 

 achromatic system invented by Dollond and Euler (which is 

 founded on the marvellous structure of the eye) by another 

 process, in order to ascertain to which the preference should 

 be given. I believe it will be sufficient to consult a work, the 

 title of which is, Dioptricce, auctore Euler, Petropoli, 1769, 

 1771, and the Analytiche Dioptrick, Leipzig 1778, or better 

 to compare a good achromatic microscope made according to 

 Eider's system, with the best instrument constructed by that of 

 Mr Airy. 



