424 Tr nsactions of the Society. 



screens are made by grinding cover-glasses to a fine grain and then 

 bringing them to the exact condition required by rubbing a little 

 white wax over the surface.* In this way the scattering power of 

 the surface can be regulated with some nicety ; since it is easy to 

 put more wax on or rub it off, until the exact condition is reached 

 which gives the desired angle to the emitted light. But it will 

 not be supposed that I can presume to recommend this as being 

 the best plan of preparing these screens. I can only say that it 

 answers. Probably other forms can be devised that will answer 

 better. This is the best that I have thought of. 



(3) The subject last discussed leads very naturally on to the 

 discussion of the illumination of the microscopic field. It has 

 been pointed out above (p. 412) that unequal and most mischievous 

 illumination results from the overlapping of antipoints intruding 

 from the margin into the centre of the field when the marginal 

 beams have suffered mutilation. But as the subject is foreign to 

 Helmholtz' paper — although very germane to his theory — it could 

 not there be pursued. I propose to return to it now in order to 

 point out the expedients by which this inconvenience may be 

 avoided. 



Working Aperture. 



Foremost among these stands the expedient to which Mr. E. 

 M. Nelson has drawn attention, of leaving a ring of "unoccupied 

 aperture " in the back lens of the objective. This, of course, can 

 only be effected by cutting down the numerical aperture of the 



admitted beam, but when it is once realised that s — ; is not 



2 sin u 



anywhere near the resolving power of a thoroughly corrected lens 

 this will cease to be regarded as a formidable proposal. Moreover 

 as, whether he likes to do so or not, the microscopist is obliged to 

 shut his condenser down below the apertometer measurement of 

 his objective if he wishes to obtain the best result from his instru- 

 ment, the rule thus laid down must, with whatever protests, be 

 recognised in practice. 



Dark-field Illumination. 



But another expedient which does not involve any loss of 

 angle in the objective, and by which very fine resolution may be 

 obtained, is that of dark field illumination. Here the marginal 

 parts of the field emit no light and consequently there are no 



* Further experiment shows that lanoline or such-like grease is better than white 

 was for this purpose. White wax absorbs no little light. Glass ground to the exact 

 grain required is better still. 



