ANOPTRAL MICROSCOPE 



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Fig. 1. In the left-hand picture living yeast cells were photographed by means of an ordinarj^ micro- 

 scope objective (Reichert achromat 60X); in the picture to the right the same specimen is shown with 

 the objective treated as described in the text. In the latter picture the cells with a good nutritional 

 status appear dark while others remain more or less pale (old, degenerating, poorly nourished cells). 

 There are no halo effects around the cells as there would be with an ordinary phase contrast. 



by the multiplicity of structures. There are, 

 however, difficulties in the way of studying 

 unstained preparations. 



All those who have worked with the micro- 

 scope must be aware of the fact that a nor- 

 mal stained biological preparation provides 

 an excellent picture when viewed through a 

 microscope with large objective and con- 

 denser apertures, whereas in the absence of 

 staining very little if anything can be seen 

 since the details are only distinguishable 

 then by their refractive power. Not until 

 the illuminating aperture has been reduced, 

 e.g., by stopping down the iris diaphragm, 

 do the details begin to appear one after an- 

 other, but even then they are surrounded by 

 concentric diffraction fringes which have a 

 detrimental effect in that they reduce the 

 resolving power. 



Nor does dark-field microscopy help very 

 much. Abrupt changes in the refracting 

 properties of the details of the object cer- 

 tainly show up clearly, but small differences 



or gradual changes in the refractive index 

 remain invisible. 



These drawbacks led the author to under- 

 take extensive experiments with the "schlie- 

 ren" method discovered by Topler nearly a 

 hundred years ago, and to apply them to 

 microscopy. It was soon discovered that an 

 annularly defined aperture in both the 

 condenser and the objective was the most 

 suitable one. The realization of annular de- 

 fined apertures in objectives, how^ever, was 

 very difficult. After experimenting with thin 

 coatings of silver, without much success, the 

 author tried coatings of soot applied by re- 

 peatedly exposing the surfaces of an objec- 

 tive lens to a candle flame. 



A microscope modified on the above lines 

 provided much better images of living speci- 

 mens viewed through it. The obvious course, 

 then, was to proceed on purely empirical 

 lines. ^ It was soon evident that excellent re- 



1 It is interesting to note that Leeuwenhoek 

 was not familiar with the theory of optics. He was 



477 



