PHYSICAL ASPECTS OF IMAGE FORMATION 57 



consider a diatom: pleurosigma angulatum (Fig. 1.73(a)). The micro- 

 scope eyepiece is removed and the objective focal plane observed by 

 means of a focusing telescope. With an objective of N.A. = 0-85, 

 and the condenser abnost fully diaphragmed, only the direct image 

 5*0 is visible (Fig. 1.74(a)). Let us put back the eyepiece and observe 

 the imaged diatom: the aspect is that of Fig. 1.73(b): the diatom is 



Fig. 1.74. Diffracted light in the focal plane of a microscope objective (pleurosigma 



angulatum) . 



not resolved. Let us consider again the objective's focal-plane and 

 open the condenser diaphragm. The spectra outside the field spread 

 out and a time comes when the spectra edges penetrate into the ob- 

 jective (Fig. 1.74(b)) and the imaged diatom is resolved. 



Under normal working conditions, perception of discrete objects 

 of high contrast and that are small in relation to the diffraction disk of 

 the objective varies but slightly with condenser aperture : illumination is 

 nearly always coherent. However, when the condenser is diaphragmed 

 too much, images do not exhibit an attractive appearance. Increasing 

 the condenser aperture improves image sharpness and subdues diffrac- 

 tion fringes. Naturally while the condenser N.A. affects but Httle the 

 perception of minute discrete objects it does take effect on the sepa- 

 ration hmit of closely adjacent objects. Images are finer and diffraction 

 fringes fade as the aperture of the condenser increases, the more 

 utmost definition is improved (Fig. 1.71). 



The numerical data mentioned in the foregoing paragraphs are 

 tabulated below. The data included in chart 1.1 apply to a perfect 

 instrument and should be considered as the very best obtainable. 

 Such results may be approximated by using, in monochromatic fight, 

 a microscope with stray light excluded to the utmost and whose 

 objectives are well corrected from spherical aberration. Chart 1.2 

 shows the chromatism effects on perception fimit of a small dark disk 



