THE MICROSCOPE. 



125 



The violet component is bent most, and hence, is brought 

 to a focus at a different point from the red ray, which is 

 bent the. least. The result is 

 a fringe of colors. This is 

 designated as chromatic aber- 

 ration. 



It is necessary, therefore, 

 to correct the chromatic and 

 spherical aberration, in order 

 to obtain the best optical re- 

 sults. The spherical aberra- 

 tion is partially corrected by 

 means of stops or diaphragms, 

 which hold back the peri- 

 pheral rays, and allow only 

 the central ones to pass 

 through, since these give rise 

 to an almost flat image. Such 

 diaphragms are shown in Fig. 

 17. The use of flint glass still 

 further, serves to correct this 

 error. 



The chromatic aberration 

 is more diiificult to correct. 

 The first successful attempt 

 at the correction of chro- 

 matic aberration in an objec- 

 tive, was made by an Italian, 

 Marzoli, in 1811. In other 

 words, 200 years elapsed be- 

 tween the discovery of the 

 compound microscope and the 

 correction of the most serious 

 defect in the working of the 

 objective. This early work, however, seems not to have 

 attracted much attention, and Chevalier, of Paris, was given 



Fig. 17. Principle of the compound mi» 

 croscope (Carpenter). F— Object in focus, 

 above this an objective with diaphragm; 

 AB— Real image of F, in opening of dia- 

 phragm; above this a compensation ocular 

 vfhich magnifies the real image AB, thus 

 forming the virtual image CD. 



