276 PROCEEDINGS OF THE QUEKETT MICROSCOPICAL CLUB. 



greatest working aperture with the highest powers an immersion 

 condenser of N.A. 1-3 must be used. An immersion condenser 

 is not very sensitive to variations of slip thickness. With a 

 very large working aperture contrast in unstained transparent 

 objects is reduced to a minimum, nevertheless the capacity of 

 a really good objective is utilised to the utmost. Mr. Merlin 

 said that it was highly important that the image of the radiant 

 should be structureless, as is the case with the lamp flame ; he 

 objected to the thorium disc on the grounds that its granulated 

 surface might give an image that could be mistaken for structure 

 in the object. Since his return to England he finds the same 

 carelessness as to illumination on the part of most workers as 

 used to be fifty years ago. Abbe condensers are used and the 

 ground-glass incandescent filament lamp is very popular and very 

 unsuitable. Mr. Merlin said that probably the great initial 

 difficulty in observational microscopy is that of recognising the 

 " critical " image. The eye must gradually become accustomed 

 to the often faint but clearly outlined images, which are such a 

 contrast to the strong diffraction effects generally preferred even 

 by microscopists of long standing. In addition to the photo- 

 graph of A. fellucida, Mr. Merlin exhibited one of P. angulatum 

 and also one of Nitzschia singalensis ; the latter showing the striae, 

 and taken with an achromatic oil-immersion objective of N.A. 1'18, 

 the lowest aperture with which this diatom has been resolved. 



Commander Ainslie said that he was glad Mr. Merlin had laid 

 such stress on the large illuminating cone and the faint image, 

 but he could not agree that the lamp flame was the only suitable 

 illuminant. No one could tell us what the correct microscopical 

 image was, it could only be judged by mathematical theory. 

 No theoretical reason had ever been given for focusing the illumi- 

 nant on the object, but unless it was so focused, if the illuminant 

 was small, it was impossible to be certain that the back lens of 

 the objective was filled with light, although it might appear to 

 be so on looking down the tube. Commander Ainslie said that 

 it was possible to see as much but no more with the flame edge 

 as with other illuminants. He attributed the remarkable dis- 

 coveries of fine structure made by Mr. Nelson and Mr. Merlin not to 

 their use of the flame edge, but to their unusually keen eyesight. 



The meeting closed with a very hearty vote of thanks to 

 Mr. Merlin for his paper. 



