166 MICROSCOPIC VISION. 



This becomes an important point when dealing with the 

 limit of microscopic vision, because the question will natu- 

 rally arise, What limit, the " black " or '^ white dot " limit? 

 For obviously there must be two limits. 



There is another, and perhaps a more important, question ; 

 viz., Which of the two is the more correct picture ? More- 

 over, as these images occur at diiferent foci, a further 

 question arises, What is the correct focus ? and on this 

 hangs the last question, for the adjustment of the objective 

 depends on the focus. What is the correct adjustment ? 



We now come to the last point ; viz., dark ground illumina- 

 tion. This illumination is best obtained by placing an 

 opaque stop at the back of the condenser, to stop out an 

 axial cone of greater aperture than that of the objective on 

 the nose-piece. Now it is found in practice that with this 

 kind of illumination, when the ground is strictly dark, 

 that the resolving limit of all objectives is lowered ; the 

 diffraction theory, however, offers no explanation of this 

 phenomenon. 



When the stop at the back of the objective is hardly 

 large enough, the ground assumes a pearly appearance ; 

 in this case the resolving limit is at its maximum. 



You have been detained long enough over this technical 

 and very dry subject, but the remarks made this evening 

 will not have been in vain if they induce any member of 

 your Microscopical Society, upon whom Dr. Fripp's mantle 

 may have fallen, to take up this subject and work out the 

 answers to these questions. Any one doing so will, I am 

 sure, earn the gratitude of all genuine students of " Micro- 

 scopy." 



