THE MICROSCOPE, 33 



time in the hands of persons who were not aware af their full 

 value, and Mr. Hall himself paid the debt of nature without 

 revealing the secret of their construction. 



In 1747, we are told that Euler suggested the construction 

 of achromatic object glasses, a problem which for a long 

 time agitated the learned in England, Holland, Italy, and 

 France ; and in 1774 he proposed the application of an 

 achromatic combination for the object glasses of micro- 

 scopes. Our countryman, DoUond, " on the faith of Sir 

 Isaac Newton's conclusions, zealously denied the possibility 

 of doing what Euler proposed, but, nevertheless, commenced 

 a series of experiments, beginning with that which had led 

 Sir Isaac Newton to his unfavourable opinions, and which 

 ended in accomplishing all that Euler had declared and 

 Newton had hoped to be possible. These experiments, which 

 included the spherical as well as the chromatic correction, 

 were completed in the year 1757, and the glory of achieving 

 this most valuable result is in no respect lessened by the fact, 

 of which there is now no doubt, that a chromatic correction 

 had been, to some extent, produced in the year 1733, by Mr. 

 Chester More Hall."* Although DoUond constructed many 

 achromatic telescopes, he did not apply the same principle to 

 microscopes ; but those which he sold were only modifications 

 of the compound instrument of Cuif. Chevalier tells usf 

 that there exists a very rare work, published at St. Petersburg 

 in 1774, under the following title: — Detailed instruction for 

 carrying lenses of all different kinds to a greater degree of per- 

 fection, with the description of a microscope which may pass for 

 the most perfect of its kind, taken from the dioptric theory of 

 Leonard Euler, and made cmnprehensible to workmen by Nicholas 

 Fuss. It contains a description of the object-glass of the 

 microscope, of which the following is the substance : — " The 

 object-glass will be composed of three glasses ; the first and 

 third of which will be of crown-glass, and the second of flint 

 The focal distance will be half-an-inch, and the aperture of 



* A. Ross. Prcwtical Illustrations of the Achromatic Telescope. 



Parti., p. 11. 



t Op. at. p. 86. 



3 



