HISTORY OP THE MICROSCOPE. 11 



were very thick, and of long focus. Although such con- 

 siderable improvements had taken place in the making of 

 achromatic object-glasses since their first discovery by 

 Euler in 1776, we find, even at so late a period as 1821, 

 M. Biot writing, "that opticians regarded as impossible 

 the construction of a good achromatic microscope." Dr. 

 Wollaston also was of the same opinion, " that the com- 

 pound instrument would never rival the single." 



In 1823, experiments were commenced in France by 

 M. Selligues, which were followed up by Frauenhofer in 

 Munich, by Amici in Modena, by M. Chevalier in Paris, 

 and by the late Dr. Goring and Mr. Tulley in London. To 

 M. Selligues we are indebted for the first plan of making 

 an object-glass composed of four achromatic compound 

 lenses, each consisting of two lenses. The focal length of 

 each object-glass was eighteen lines, its diameter six lines, 

 and its thickness in the centre six lines, the aperture only 

 one line. They could be used combined or separated. 



A microscope constructed on this j)rinciple, by M. Che- 

 valier, was presented by M. Selligues to the Academie des 

 Sciences on the 5th of April, 1824. In the same year, and 

 without a knowledge of what had been done on the Con- 

 tinent, the late Mr. Tulley, at the suggestion of Dr. Goring, 

 constructed an achromatic object-glass for a compound 

 microscope of nine-tenths of an inch focal length, com- 

 posed of three lenses, and transmitting a pencil of 

 eighteen degrees ; this was the first that had been made in 

 England. 



Sir David Brewster first pointed out in 1813, the value 

 of precious stones, the diamond, ruby, garnet, &c., for the 

 construction of microscopes. " The durabilit}^," he says, 

 " of lenses made of precious stones is one of their greatest 

 recommendations. Lenses of glass undergo decomposition, 

 and lose their polish in course of time. Mr. Baker found 

 the glass lenses of Leeuwenhoek utterly useless after they 

 became the property of the Royal Society. The glass 

 articles found in Nimroud were decomposed, while the 

 rock crystal lens was uninjured." Mr. Pritchard at one 

 time made two plano-convex lenses from a very perfect 

 diamond, one the twentieth of an inch focus, which waa 



