48 ; ICROSCOPIC ILLUSTRATIONS. 



late Mr. Tulley have succeeded in producing achromatic 

 object-glasses. 



In the same work, the amateur is made acquainted 

 with a class of curiously organized bodies, which re- 

 quire a certain degree of perfection in a microscope to 

 render their structure visible, and thus enabled to exercise 

 a salutary check over the workman. Since this work 

 appeared, the achromatic microscope has been elevated 

 to such a pitch of excellence in this country that it stands 

 unrivalled throughout the world. We can now construct 

 object-glasses with angles of aperture of great extent, 

 and with the aberrations so accurately neutralized, that 

 hardly any thing remains but the colouring from the se- 

 condary spectrum, or that which is occasioned by the 

 irrationality of the dispersive powers of the different 

 media employed in their construction. It is not to be 

 expected that much more can be done ; unless, indeed, 

 other substances could be found for the making of lenses, 

 such as will obviate those defects, which it is impossible 

 for the workman to remedy with his present materials. 



Many valuable and interesting additions to microscopic 

 science have been made from time to time by Sir David 

 Brewster; these should not be passed over without 

 proper notice; but since Sir David has so ably described 

 them himself, it is far more becoming in me to refer the 

 reader to his works, than to enter upon an explanation of 

 their merits in this place. 



Our observations have been hitherto directed almost 

 exclusively to the optical portion of microscopes, and 



