Notes on Resolution. 377 



There was a further difference in that the light from a lamp flame 

 more than half an inch deep may not produce the same result as 

 a source of illumination which is almost a surface at right angles 

 to the plane of illumination. This would not, however, explain 

 the discrepancy in my observations, and experiment 2a pointed 

 to the idea that the size of the area of object illuminated might 

 influence the question. I therefore placed a negative lens com- 

 bination of a focal length of about 1 inch in front of the lamp, 

 which reduced the size of the image of the Pointolite to about one- 

 quarter of its previous size, with the following result : — 



5. 2 mm. Achromatic 1 • 3 N. A. 



Green Light. — Aperture of object-glass reduced by iris 

 diaphragm at back till 80,000 lines could be just 

 resolved with full illuminating cone. The least reduction 

 of the illuminating cone destroyed resolution. 



6. 2 mm. Achromatic 1 * 3 N.A. 



Green Light. — All conditions exactly the same as observa- 

 tion 5, except that negative lens in front of lamp was 

 removed. No trace of the resolution of 80,000 lines 

 could be obtained with any aperture in the illuminating 

 cone. 

 The result of these observations appears to show that Mr. 

 Nelson's experiments and the conclusions he drew were correct 

 for an illumination which extended over a large area of the object, 

 and that under such conditions the full aperture of an object glass 

 can never be utilized ; but that with a small portion of the object 

 illuminated, the full aperture of an object glass can be utilized and 

 further resolution obtained. I am not at present putting forward 

 any explanation of these results. A great deal of research on the 

 question is required. In the meantime it will be interesting to 

 see if others will confirm the above results. In carrying out these 

 experiments one important condition is that both the iris diaphragm 

 and the optical portion of the substage condenser should be accu- 

 rately centred, as in the resolution of lines a spurious resolution 

 can be readily obtained by even a small obliquity of the light. 



2 D 



