The Presidenfs Address. Btj Prof. P. Martin Duncan. 149 



in diameter, will necessarily appear to us, not in tlieir proper 

 proportions, but with greatly increased diameters, and that very 

 minute striations must appear as if the dark and bright interspaces 

 were nearly of equal breadth, although in reality not so. 



There are obviously many histological problems, such as the 

 question of the structure of muscle, which a proper knowledge of 

 this part of the subject may greatly help to elucidate. 



The facts which we now have before us in regard to microscopical 

 vision, are sufficient to justify the injunction of Professor Abbe that 

 " the very first step of every understanding of the Microscope is 

 to abandon the gratuitous assumption of our ancestors that micro- 

 scopical vision is an imitation of macroscopical, and to become 

 familiar with the idea that it is a thing sui generis, in regard to 

 which nothing can be legitimately inferred from the optical 

 phenomena connected with bodies of large size." That there must 

 be a great deal more yet to be elaborated in regard to the origin 

 and nature of the phenomena we have been considering, is obvious, 

 and I hope that the attention of our own physicists and microscopists 

 will be directed to a subject of such extensive practical bearing, not 

 merely to the theoretical microscopist, but to the large class of 

 practical histologists who are entirely dependent upon the Microscope 

 for the accuracy of their observations. 



The Aperture of Objectives. 



The "aperture question," as we all know, gave rise, several 

 years ago, to a somewhat acrimonious controversy, not in the 

 * Proceedings ' of the Society, but in the unofficial section of its then 

 Journal, and doubtless there were some Fellows who, at the 

 beginning of last year, regarded with no little apprehension the 

 prospect of a revival of that controversy. But, notwithstanding the 

 warmth with which it was debated in its new form, no one will, I 

 am sure, deny the very great value that the renewal of the discussion 

 — between Mr. Crisp and Mr. Shadbolt — has been in bringing to the 

 light what had previously been confined to a few. If any one does 

 not now comprehend how an immersion objective can have an 

 aperture greater than that of a dry objective of 180°, at least it 

 cannot be any longer charged against this Society, that means have 

 not been provided to enable him to do so. 



The essential difference between the old and the new view of 

 aperture is simply, that the former considered only the rays which 

 enter the objective, while the latter deals with those which emerge 

 from it. 



The disadvantage of the former method, which estimated the 

 incident pencils entirely by their angles, has been its inevitable 

 tendency to give a fictitious importance to the angle of the entering 



