14 



ELEMENTS OF /APPLIED MICROSCOPY. 



part or periphery of a biconcave lens are brought to a focus 

 somewhat closer to the lens than those which lie nearer 

 the optic axis. This deflection and the consequent blurring 

 of the image increase with every increase in the curva- 

 ture of the lens. A still more serious defect lies in the 

 fact that the rays of light of different colors are differ- 

 ently affected by the ordinary lens, those of shorter wave- 

 length at the violet end of the spectrum, (v) , coming to a 

 focus first, as indicated in Fig. 12. The result is that 



Fig. 12. — Chromatic Abeesation. (After Hager-Mez.) 



rings of spectral colors are produced bordering the ob- 

 jects examined; not serious in the simple magnifiers, 

 this effect is greatly exaggerated by passing through the 

 long distance between the objective and the eyepiece of 

 the compound microscope. For two centuries after the 

 construction of these instruments they were so subject to 

 spherical and chromatic aberration as to have little prac- 

 tical value, and were on the whole inferior to the best 

 simple microscopes. 



10. The Achromatic Objective. — The third decade of 

 the nineteenth century saw these difficulties overcome. 

 It had long been recognized that in different sorts of glass 

 the refractive or bending power and the dispersive power 



