397 



NOTE. 



Resolution of Amphipleura pellucida. 



Mr. A. Eliot Merlin and Mr. E. M. Nelson write that they have 

 resolved the transverse striae on Amphipleura pellucida mounted in 

 Professor Hamilton Smith's medium, illuminated by sunlight passed 

 through an acetate of copper screen. Powell's chromatic condenser 

 with oblique slot was used, oil-immersed to the slide, the objective 

 being Powell's dry apochromatic j, and the power only 350 dia- 

 meters. They think that this is probably worth recording, as the 

 resolution must be very strong and sharp before it can be seen with 

 such a low magnification. 



Later they found that the striae on A. pellucida mounted dry on 

 cover could be distinctly resolved into dots with the vertical illu- 

 minator and Leitz Yo m -> 1*3 N.A. The important point in this 

 observation is that the illumination was central, by which term it is 

 meant that the annulus beyond N.A. 1 '0 at the back of the objective 

 was evenly illuminated all round. This is remarkable, because it was 

 formerly understood that, in order to obtain the maximum resolution, 

 it was necessary to utilise only a small part of the annulus in a 

 direction at right angles to the striation. "When, however, this latter 

 arrangement was tried, the structure remained unresolved even with 

 an N.A. of 1*4. 



In each instance the edge of the flame was maintained at such a 

 distance from the vertical illuminator that its image was sharply 

 focussed upon the object. 



