284 the president's address. 



no advance on the old — in fact, that it is retrograde — because of this 

 limited diameter of true definition ; and they point out that they 

 possess old-fashioned ones of far more extensive diameter of field 

 in what they call perfect focus. To such as these I must say 

 that, according to existing knowledge, I believe no living computer 

 can make an objective of high power which will give really 

 perfect definition over the outer-lying portions of the field with- 

 out a falling off in the central and intermediate zones. In fact, 

 the whole thing is a compromise. If you have the best in the 

 centre and intermediate, the outer zone must suffer ; whereas if 

 all three are to be in focus at the same moment, it is at the 

 expense everywhere of the finest definition. Seeing that most 

 microscopical objects only occupy the central and intermediate 

 zones of the field of view, it is generally admitted to be better to 

 make these zones furnish the finest possible definition, even 

 though it be at the expense of the outer one, this being the 

 reason why modern objectives show the falling off in the outer- 

 most parts of the field to which I have referred. 



So much for the optical side of the question ; now as to what 

 may be termed the mechanical. There is no doubt that for the 

 past several years the short-tube instrument has been steadily 

 gaining favour, and that somewhat rapidly. If you take the 

 trouble to investigate the matter, you will find that nearly every 

 new stand or objective is of the Continental length, and hardly one 

 of the English model. With respect to the student, the greater 

 portability of the one over the other is a great inducement, and 

 with those who have to take their instrument about with them, 

 specially those going abroad, it is imperative, provided that the 

 utility of the optical parts be not impaired thereby. With the 

 biologist, and most certainly with the bacteriologist, the short tube 

 is much preferred, because it permits the worker to sit at his 

 bench and use his instrument upright — a necessity in some 

 observations — which he cannot do with the long tube unless 

 provided with an office stool, or some such elevated seat. 



