and on (he Memoir of Euler. 239 



ration is well corrected, it is of small moment whether the chro- 

 matic is so or not. — I must observe that (he necessity for re- 

 moving dispersion is never so sensible us when (he spherical has 

 been withdrawn ; for the latter covers and conceals the former, 

 just as the taste of one sapid substance will cover that of an- 

 other, or, speaking accurately, as a fog conceals the tints of a 

 landscape. 



This set of object-glasses teas exquisitely centered and ad- 

 justed. 



D. Proof-objects have been wretchedly abused by dishonest 

 tradesmen, owing to the vast difference of facility of resolution 

 which exists between specimens of the same kind, or nearly so, 

 so that it has been weil observed by Mr Lister, that the only 

 safe way to determine between two microscopes, is to apply the 

 same identical sccde to both, under the same circumstances. 



In illustration of this position, I shall observe, that I have 

 seen a specimen of French podura of no more difficult resolu- 

 tion than the menelaus ; yet not to be distinguished from the 

 difficult English podura, by its outward and visible character. 

 Even among the most difficult English podura there are fre- 

 quently a few large coarse scales to be found of very easy de- 

 monstration. The same discrepancy is found among the other 

 kinds of lined objects. 



E. See my paper on monochromatic light. 



1*". This is a very excellent set of object glasses for practical 

 purposes, but neither of the sextuple compositions are so per- 

 fect as that which I shall next describe ; in order to arrive at 

 perfection, it will, I believe, be found necessary to have Jixcd 

 combinations. My idea of a perfect achromatic microscope 

 would be this, it should have only one achromatic eye-piece, 

 and that to be as shallow as possible, all the different powers 

 to be obtained by changing the object-glasses of which there 

 should be, if only of the double kind, about half a dozen lixed 

 Compositions or systems ranging from two inches focus to the 

 utmost shortness consistent with perfection ; in short, the in- 

 strument would be pretty much like a compound microscope 

 of the old plan, furnished with achromatic systems of object- 

 glasses. 



(i. This set is about ,'jli of an inch focus, and ha an an- 

 XKW si IMI... VOL. v. no. ii. OCTOB1 H 1831. y 



