18 



PROGRESS IN MICROSCOPY 



Curve (b) corresponds to a fluorite objective: the first minimum, 

 close to 20 fi is more prominent: decreased image contrast may, 

 therefore, be expected. In all these curves, the central disk, acting 

 in fact as image, has a diameter of 20 microns approximately. 

 Anything outside it is detrimental diffracted light. In an achromatic 

 objective, the detrimental diffracted light is particularly marked and 

 a drop of the central peak is evinced. Perception limit of small details 

 is consequently much less satisfactory in an achromatic objective. 

 Numerical values are shown later. On the other hand, the all-important 

 colour of the diffraction pattern is not evinced in the curves shown 

 in Fig. 1.20. The central disk, in both a perfect or an apochromatic 

 objective, is definitely white. Only the first luminous ring exhibits 

 a slight iridescence. Iridescence is more prominent in the fluorite 

 objective and, in the achromatic objective the slightly coloured central 

 disk is ringed by diffracted light whose colour is vivid (usually greent 

 purple). As in achromatic objectives, the amount of diffracted ligh- 



^'0 5 



la) apochron.otic oDiective 



lb) Fluorite oDieciive 



10 20 ^0 40 'SO 60.,- 



Ic) Achromatic oDiectiuf 



Fig. 1.20. Diffraction patterns: (a) — apochromatic objective; (b) — fluorite objective; 



(c) — achromatic objective. 



outside the central disk is important and its detrimental effects on 

 a coloured object can be readily visualized. Achromatic objectives 

 should not be used, therefore, in colour photomicrography. 



The foregoing performance shows the difference in quality between 

 apochromatic and other lens-type objectives. The first bright ring, 

 shown by an apochromatic objective, scarcely exceeds the intensity 



