374 Trmisactio'its of the Society. 



corrected achromatic 1 * 3 N. A. condenser, soliil cone about 1 • N".A., 

 ,L;ave no resolution wiili light of any colour on specimens mounted 

 in styrax or monobromide of naphthalene. With specimen in 

 realgar and dark blue light, resolution just glimpsed, but not 

 actually seen, but signs of structure are visible. 



With dark ground illumination and specimens in either real_i;ar, 

 styrax or monobromide, resolution was thoroughly distinct with 

 blue and green light. In realgar it was thoroughly black with 

 blue light, good with green, and faint with white light, not visible 

 with red light. 



The possibility of the application of these observations to 

 medical microscopy is evidenced by the examination of stained 

 slides of such every-day objects as anthrax, tubercle and malaria, 

 by dark ground illumination in combination with colour-filters. 

 Anthrax stained so that by transmitted light it appears blue, when 

 viewed by opaque illumination and a purple screen which transmits 

 only blue and red li^jht reflects red and appears of a blood-red 

 colour. Under similar conditions tubercle stained red appears by 

 reflected light a light green. The tubercle stands out much more 

 vividly than is the case with transparent specimens. Indeed, the 

 ' way in which stained bacteria mounted in balsam stand out from 

 their surroundings, quite apart from their colour, suggests that 

 they have a very different refractive index to balsam. A specimen 

 of the signet ring form of malaria in a similar manner is very 

 distinctly differentiated from the blood corpuscle in which it is 

 encysted. 



The question of contrast, glare and flooding in transmitted 

 illumination are subjects on which I am making a series of 

 observations, and I hope at some future date to report some 

 results. 



B. The Influence of the Area of Illumination on the 

 Resolution of Grayson's Rulings. 



Mr. E. M. Nelson has demonstrated that for the resolution of 

 diatoms and G-ray son's rulings with a solid axial cone of light, the 

 finest resolution was obtained when the illuminating cone of light 

 was not as large as the full aperture of the object glass, but was 

 about 1^ or I of the maximum angle. Amongst others, some time 

 ago I repeated and confirmed his experiment. In recent work 

 with an 8 mm. Achromatic and an 8 mm. Apochromatic. in both 

 of which all zones of the lenses were more perfectly corrected than 

 has generally been the case, I was unable to confirm the well- 

 known experiment, and I have made a large number of observa- 

 tions with lenses of different powers to investigate the matter. 



These observations were made with a 100-candle power electric 



