APPENDIX. 293 



natural philosopher, Sir David Brewster, suggested that 

 diamonds should be used in the construction of simple 

 microscopes. Pritchard of London, and C. Chevalier 

 and Oberhaiiser of Paris, made several attempts to 

 carry this suggestion into eiFect, and they constructed 

 several lenses of precious stones. The results, however, 

 did not answ^er the expectations which had been enter- 

 tained, and it was soon found necessary to relinquish the 

 use of such costly materials. Opticians were then led to 

 reconsider an idea which had been repeatedly throwni out, 

 but which it remained for Wollaston to deveiope with his 

 usual talent. This idea consisted in the superposition of 

 several lenses, and at last, in 1829, C. Chevalier, follow- 

 ing the principle of the English philosopher, constructed 

 his doublet, which yields to the observer almost ail the 

 advantages that can be obtained from a simple microscope. 

 This instrument consists of a compound lens of two 

 plano-convex glasses separated by a diaphragm, and 

 whose plane sides are both directed towards the object. 

 This lens is supported by a small solidly fixed beam, 

 whose horizontal arm may be turned in every direction 

 above the object. 



The history of the compound microscope exhibits 

 far less continuous progress than that of the simple 

 microscope. This instrument, which was, very pro- 

 bably, invented by Galileo *, in 1612, or perhaps by 



* This illustrious astronomer, -whose name is one of the most 

 familiar in science, was born at Florence in 1564, and died in 

 1642. From his youth he had devoted himself to mathematics, and 

 he lectured on this science at the Universities of Pisa and Padua. 

 Having heard of the discovery of the telescope in Holland, Galileo 

 immediately tried to construct an instrument of the kind, and soon 

 invented the telescope which still bears his name. With this 

 instrument he discovered the phases of Venus, the Sun's spots, and 

 Jupiter's satellites. The result of his astronomical discoveries 

 u 3 



