and the Achromatism of Microscopes. 229 
Huygenian eye-piece, is now in general use for astronomical and 
reflecting telescopes. 
While the eye-piece of Huygens was used for astronomical 
purposes, the eye-piece of three lenses, each contiguous pair being 
placed at a distance equal to the sum of their focal lengths, as 
described in our elementary works, was still used for erecting the 
image. The first notice that we find of any alteration in the 
erecting eye-piece, is contained in a letter from Dollond to Short, 
printed in the Phil. Trans. for 1753. In this he stated that he 
had made achromatic eye-pieces with 4 and 5 lenses for some time, 
and that, finding some of them to have reached the Continent, 
he considered it expedient to assert his claim to the invention 
before it was likely to be disputed. It appears that his object 
at first was merely to diminish the spherical aberration, on the 
same principle on which it had been attempted by Huygens: 
but that, finding that the chromatic aberration might also be 
corrected by the same construction, he had combined 4 and 5 
lenses, with particular attention to the correction of colour. It 
would seem from this letter, that he had then no theory which 
enabled him, generally, to make his eye-pieces achromatic: and 
it is not known whether he afterwards made use of any formula. 
It is remarkable that in this letter he speaks of an achromatic 
object-glass, as a thing totally to be despaired of. 
The attention of philosophers was at this time very generally 
turned to the consideration of chromatic aberration. Six years 
before that letter was written, Euler, from a hint given by Newton, 
had conceived the idea of making a lens truly achromatic. Two 
years after its date, Dollond found that it was possible to make 
an achromatic lens. In the next year, 1756, Clairaut gave, in 
the Memoirs of the French Academy, his first investigations re- 
lative to achromatic object-glasses: which he continued for several 
years. They were simplified by D’Alembert, in some papers first 
GG2 
