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XVII.— NOTES ON EESOLUTION. 

 By Conrad Beck, C.B.E., E.K.M.S. 



A. The Influence of Colour-Filters on the Resolution of 

 " Bark Ground " Illuminated Objects. 



1 HAVE been devoting considerable time to working with dark 

 ground illumination under different conditions, and I have found 

 several interesting points which bear on microscope resolution. 

 Such work can scarcely be done without a focusing illuminator, 

 as it involves the examination of many different mounted 

 specimens. 



It has been stated that the resolution by dark ground illumina- 

 tion is not equal to that of transparent illumination — I cannot 

 confirm this. Under some circumstances, if not all, I find it fully 

 equal and far more easily accomplished. 



A good example is the resolution of Plcurosigma angulatum 

 into dots with an 8 mm. achromatic object glass with a N.A. of 

 only • 5. Unless the zones of this lens were almost perfectly 

 corrected I doubt if it could be resolved with so small an aperture. 

 Carpenter mentions 0*65 N.A. as the smallest aperture with 

 which P. angulatum has been resolved. 



With transparent illumination and a condenser with a cone 

 the full • 5 N.A. and a X 50 eye-piece, it cannot be resolved with 

 red or orange light, rather faintly with green, and brilliantly v/ith 

 blue light. 



Sucii an object is therefore on the limit of resolution of this 

 lens and on the theoretical limit of this aperture, and it forms a 

 good test of resolution by dark ground illumination. 



The resolution with dark ground illumination is far more 

 brilliant than with transmitted light. With transmitted illumina- 

 tion I failed to get resolution unless the diatom was mounted in 

 realgar or dry. With the dark ground illumination the resolution was 

 also perfect with specimens in styrax or monobromide of naphtha- 

 lene. I could not, however, get any resolution with red or orange 

 light. It would therefore appear that dark ground gives equal but 

 no greater resolution, but much greater contrast, so that diatoms 

 practically unresolvable because of lack of contrast by transmitted 

 light are readily resolved by dark ground illumination. 



A further example of resolution with dark ground illumination 

 with a high power object glass gives the following results : — 



A 1/8 (3 mm.) oil immersion Beck achromatic object glass, 

 0*95 N.A., on Amphipleura pellucida, light " Pointolite," perfectly 



