42 MICROSCOPIC ILLUSTRATIONS. 



this moment a triple object-glass, evidently made in 

 imitation of one of Mr. Tulley's achromatics. In con- 

 structing it, however, the optician, a person of great 

 respectability, was so unacquainted with what an achro- 

 matic is designed to effect, that he actually placed a stop 

 behind the lenses, so that, notwithstanding the focal 

 length does not exceed half an inch, an angle of aperture 

 of only 7 degrees is obtained. This object-glass of course 

 is inferior to a common lens : and hence has originated 

 the erroneous notion that the introduction of achromatics 

 has been no improvement to microscopes. 



Should the question arise, why a single lens, or a com- 

 bination of convex ones, cannot be mounted so as to have 

 an angle of aperture equal to that in achromatics, the 

 answer is, that it can be done : I have often done it ; and 

 it affords an excellent method of shewing by comparison 

 the vast superiority of the latter over the former. Long 

 experience has taught the optician that common lenses, 

 when mounted in this manner, occasion such great aber- 

 rations that there is no possibility of obtaining any thing 

 approaching to a distinctness of vision. Hence has 

 arisen the necessity of placing a small stop behind them ; 

 the effect of which being to transmit only a small pencil 

 of the light collected by the glass, the chromatism is re- 

 duced and rendered imperceptible. So effectually, indeed, 

 is this accomplished, that when one of our ablest philoso- 

 phers was told by Dr. Goring that he proposed to make the 

 object-glasses of microscopes achromatic, he exclaimed, 

 " I thought they had always been so constructed." 



