376 Traiisactions of the Society. 



2. 8 mm. Achromatic 0-5 N.A. 



Green Light. — 45,000 lines well resolved. 

 Blue Light. — 50,000 lines well resolved. Full cone of 

 illumination. Cutting down illuminating cone damaged 

 resolution, and resolution disappeared when it was 

 below 0*45 N.A. Cutting down aperture of object glass 

 so as to only just resolve 45,000 lines, the least cutting 

 down of aperture of illuminating cone below full aper- 

 ture destroyed resolution. 30,000 lines, the same result. 

 2a. Green Light — Full aperture of object glass used. Ground 

 glass placed in front of lamp. Aperture of illuminating 

 cone 1 N.A. 30,000 lines faintly visible. Aperture of 

 illuminating cone • 5 N.A. 35,000 lines faintly visible. 

 Aperture of illuminating cone 0*45 N.A. 40,000 lines 

 faintly visible. 



3. 8 mm. Apochromatic 0"65 N.A. 



Blue Light. — 60,000 lines well resolved with full aperture. 

 0*65 N.A. illuminating cone, resolution destroyed directly 

 aperture of illuminating one is reduced ; only 55,000 

 visible. 



4. 16 mm. Apochromatic 0*35 IST.A. 



Green Light. — 30,000 lines clearly resolved, but faint, full 

 aperture, 0"35 N.A. illuminating cone. The least 

 reduction of illuminating cone destroyed resolution. 

 Bhie Light — 35,000 lines resolved with full aperture cone, 

 but not if illuminating cone was reduced. 

 Observations 2, 3 and 4 are in direct contradiction to observa- 

 tions 1, 2a, and to Mr. Nelson's experiment, and I was inclined to 

 think that the perfection of the corrections of the different 

 portions of the oJojective for zonal aberrations might have some 

 bearing on the question. I therefore adjusted a 2 mm. object 

 glass so that it had a steadily progressive aberration from the 

 centre to the edge ; but beyond the fact that it had to be used 

 with different lengths of tube for lines of different fineness, and 

 did not reach so high a point of resolution, it gave no results of 

 interest. 



It was, therefore, evident that the conditions in the different 

 observations were dissimilar in some respect, and the only condi- 

 tion that I could notice was that the area of light on the object in 

 observation 1 and 2a was of a larger proportion of the field of the 

 microscope. With a X25 eye-piece and 2 mm. objective it was 

 larger than the field of view, whereas in all other cases it was a 

 small brilliant patch in the centre of the field, the rest of the field 

 being almost black. Until the introduction of the Pointolite such 

 a condition had not been easy to obtain, and had not been the 

 condition of Nelson's test, where the edge of a lamp flame which 

 more than filled the field in at least one direction was employed. 



