CHAPTER IX. 



ACHROMATIC MICROSCOPES. 



Once upon a time, at a meeting of a certain society of 

 savans, a telescopic speculum 15 inches in diameter was 

 submitted to their judgment. The surface of this spe- 

 culum was most exquisitely polished, and as the fellows 

 severally examined it they all praised its great beauty. 

 At length a well-known practical astronomer was re- 

 quested to give his opinion on this work of art, which he 

 declined doing, stating as a reason that a speculum might 

 have a very fine pohsh and yet be thoroughly useless if 

 its figure was not accurate. This remark applies with 

 equal force to the construction of the Microscope, which 

 may exhibit great mechanical skill, exquisite workman- 

 ship, and a high degree of finish, and yet for actual use 

 be a very inefficient instrument. In Chapter viii. of the 

 ' Microscopic Illustrations ' were given the first regular 

 directions for the cqnstruction of Microscopes. The 

 principles there laid down have been acknowledged more 

 or less by all our first-rate makers since its publication, 

 though each artist has varied some portion of the details 

 to suit his own views. There is therefore some difficulty 

 in selecting for the reader a form in real use possessing 



