1S8 
MR. LISTER ON THE IMPROVEMENT OF 
each in focal length, were made to screw before each other, so as to be used 
together or alone in the manner long practised with single lenses. 
The chromatic aberration was thus in a considerable degree corrected, but 
the glasses were fixed in their cells with the convex side foremost, which is 
their worst position ; and the spherical 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 com- 
bining 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 instruments, ap- 
pears to have observed the great error in the position 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 0.1 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. 
lie 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 establish- 
ment of l tzschneider 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 microscope of 
