170 Mr Lister on the improvement of the 



corrected, but the glasses were fixed in their cells with the con- 

 vex side foremost, which is their worst position ; and the sphe- 

 rical error was in consequence enormous, showing itself even 

 through the contracted opening, to which it was necessary on 

 that account to limit them. 



Yet, inferior as was the instrument of Selligue, the happy 

 idea of combining achromatic object-glasses, now generally 

 adopted, and to which their present superiority is owing, seems 

 to have occurred to no one else till put in practice by him ; and 

 the very simple structure of his glasses will be shortly seen not 

 to be incompatible with the finest microscopic vision. 



Chevalier of Paris having manufactured some of these in- 

 struments, appears to have observed the great error in the po- 

 sition of Selligue's glasses ; he retained their construction, but 

 turned their plane sides foremost ; and making them of shorter 

 focal length, and more correctly achromatic, produced in 1825 

 a microscope far superior to the former. His deepest glasses 

 are not more than 0.4 inch in focal length, and two of these 

 were united in his earlier instruments for his highest power ; 

 but this was the only combination retained in them, and all his 

 glasses were restricted to apertures too small to show difficult 

 test objects. 



He has since increased the number of his glasses to be used 

 together, and otherwise improved their performance ; but if I 

 may judge from microscopes of his which I have recently seen in 

 this country, he does not yet derive from the construction that 

 he has adopted, all the advantage which it may afford. 



I am unacquainted with the date of the first production of 

 Fraunhofer's glasses ; they resemble the French in having 

 their flint lens plano-concave, but they are not cemented, and 

 the inner surfaces are not in contact, each object-glass being 

 adapted by the curves of its convex lens for being used alone. 

 My friend Mr Brown has kindly lent me a series of five glasses 

 of this description, purchased by him at Munich a few months 

 ago from the establishment of Utzschneider and Fraunhofer, 

 which range from 1.8 to 0.43 inch in focal length, and are of 

 excellent workmanship : they screw before one another in the 

 manner of Selligue's. It appears from an account of the mi- 

 croscope of those artists, printed in 1829, that it is only lately 



