Notes on Eesolution. 375 



Pointolite lamp about 12 inches from the stage of the microscope, 

 ni) bull's-eye or mirror being used. 



An adjustable pair of neutral glass wedges was placed imme- 

 diately in front of the lamp, by means of which the brilliancy of 

 the light could be varied to an almost unlimited extent, so that 

 different intensities could be used or the same intensity could be 

 maintained when the aperture of the substage condenser was varied 

 or colour screens were interposed in the beam of illuminating 

 light. Blue and green light were produced by the use of Wratten 

 filters C and B, giving moderately monochromatic light with pre- 

 dominant wave-lengths of about 4,600 and 5,200. 



The substage condenser was as perfectly corrected as an oil 

 immersion object glass, sufhciently so as to resolve Navicula 

 rhomboides with a brilliantly black resolution. Aperture 1 • 3, 

 focal length 8 mm. The condenser was focused so that the 

 Pointolite tungsten ball was very slightly out of focus. Eye- 

 pieces used, X17, X25 and X50 compensating eye-pieces. In 

 some cases blue light was used in order to give greater resolution ; 

 in others green, on account of the unpleasant colour of the blue 

 light. 



The Grayson's rulings emplo}'ed for low powers were bands 

 which increase in fineness in steps of 5,000 between 5,000 and 

 60,000 lines to the inch ; for high powers those which increase in 

 steps of 10,000 from 10,000 to 120,000 lines to the inch. 



The following out of a number of similar observations are 

 sufhcient to illustrate the point. All the lenses were Achromatic 

 and Apochromatic lenses made by my Company, and with the 

 exception of the specially corrected 2 mm., had all their zones 

 very perfectly corrected. 



1. 2 mm. Achromatic 1 ' 3 N.A. 



Blue Liglit. — Iris diaphragm behind object glass 120,000 

 lines resolved full aperture — evidently finer lines, if 

 available, could be resolved. It thus formed no test 

 and the iris diaphragm behind object glass was, there- 

 fore, closed till only 80,000 lines could be resolved. 

 (This band was a very regular ruling, and was there- 

 fore selected for the purpose.) The iris diaphragm 

 under the condenser was then closed down till only 

 about seven-eighths of the full aperture of the object 

 glass was filled, and it was then found that the aperture 

 of the object glass could be further closed without 

 destroying resolution, and a point was reached where 

 the 80,000 band was visible with the illuminatinsr cone 

 about seven-eighths the aperture of the object glass, 

 and either increasing or decreasing the aperture of the 

 illuminating cone destroyed resolution. This entirely 

 confirmed Nelson's experiment. 



