22(j M. Chevalier's Answer to Dr Carina's 



a 



iug- rigorously the same radius, and heing joined together by 

 a substance as transparent as glass, which obliterates their sur- 

 faces. The introduction of dampness is prevented, and the re- 

 flections produced at their interior surfaces are neutralized ; 

 therefore they are much more transparent. Being, moreover, 

 invariably fixed, they can be employed with as much facility 

 as simple lenses. This it is that makes me think that this is 

 One t rue method of adjusting achromatic object-glasses, and that 

 if the practice has not been extended to all, it is because the 

 glueing has not been well performed ; if well executed, it is 

 always advantageous.* 



It is true that we were too much impressed with the idea, 

 that achromatism being once obtained, distinctness and *he con- 

 densation of an artificial light ought to be sufficient ; but after 

 your learned advice, we could observe that a large aperture was 

 also an improvement (perfectionnement, — see Remark B at the 

 end,) which must be obtained in microscopic lenses; and that 

 if the effect of object-glasses does not entirely depend upon 

 their aperture, at least, ecetcr'is paribus, the object-glasses which 

 admit the greatest opening are the best and the most pene- 

 trating. 



Since the object-glasses were sent to Mr Lister, one of those 

 systems of six glasses, composed of three object-glasses placed 

 one upon the other, each four lines focal distance, has been 

 sent to you. (See Remark C at the end.) I have studied the 

 letter in which you make some very clever remarks upon that 

 sextuple achromatic system- You say with reason that achro- 

 matic lenses must be achromatic, if it was merely on account 

 of their name. The advantage of the set in question you ad- 

 mit to reside rather in their aplanatism than in their achroma- 

 tism, because you have perceived that a little colour does not 

 alter their performance sensibly, as you have seen with them 

 the lines and lozenges on the scales of the Podura plumbea. 

 (See Remark D.) 



I say on this occasion, that it is very difficult to judge of the 

 perfect achromatism of a lens, or of a system of lenses ; for 



* The cementing of achromatic object-glasses was first executed in France, 

 together with the practice of burnishing them into settings for the purpose 

 of combination with facility. 



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