INTRODUCTORY REMARKS. 35 



chromatic aberration, were obliged to be remedied in 

 a low power by placing a diaphragm or stop behind the 

 lens, so as to reduce the aperture j by which means so 

 much of the light transmitted by the lens was cut off, that 

 the delicate structure of an object was hardly visible for 

 want of light. 



The calculations of Euler and other mathematicians 

 were soon directed towards finding out such forms for 

 single lenses as would occasion the minimum quantity 

 of aberration. Still, the microscope derived but little 

 practical advantage from their investigations ; and the 

 reason of it I apprehend was this : their formulae were 

 all calculated for lenses of very small angular aperture, 

 whilst the goodness of a lens for the microscope depends 

 upon the opposite extreme. 



In 1824, in consequence of Sir D. Brewster's remarks 

 on the diamond, I commenced making lenses of precious 

 stones*. These, when the stones are clear, free from 

 flaws, and from double refraction, are the most perfect 

 single magnifiers we at present possess. 



From the period just named, a new era sprang up in 

 the history of the microscope. In 1829f, Dr. Wollaston 

 sent forth to the world his doublet lens, which consisted 

 of two plano-convex glasses. This invention, the result 

 of experiment, caused a most astonishing alteration for 

 the better in the constructing of microscopes. It sug- 

 gested the idea of employing more than one glass in 



* See Microscopic Cabinet, chap. 14. 

 f Philosophical Transactions, 1830. 



