THE PRESIDENT'S ADDRESS. 283 



because the lower focal planes of each lie exactly the same 

 distance down the draw-tube. 



This slight digression leads me quite easily to the point. It is 

 evident that at 180 mm. the magnification of the image produced 

 by the 2-nim. is only 90, whilst at 270 mm. it is 135 ; hence it is 

 obvious that the amplification of the short-tube objective requires 

 more magnification by the eye-piece than one of a similar focus 

 on the long tube — which is the same as saying that the ocular has 

 more work to do on the short-tube system than on the lone. 

 This proves that the contention of the older microscopist was a 

 valid one, and not a simple fancy, so far as it went ; and I trust 

 I have made the matter clear in its details. 



Since the introduction of the Jena glass, however, opticians have 

 been able to construct objectives which can stand eye-piecing 

 in a way that the old school never dreamed of, even in their 

 moments of wildest imagination ; for you know as well as I do 

 that a modern lens — a semi-apochromatic, I mean — that will not 

 work successfully with an eye-piece that raises the final magnifi- 

 cation of the object to a figure equal numerically to 1,000 times 

 its numerical aperture, is not worth the box which contains it. 

 This being true, which it undoubtedly is, the great objection is 

 at once nullified, and the controversy is placed on a different 

 footing. 



As regards the relative diameter of the tubes in the two 

 forms of make, the English type, being slightly larger, requires all 

 the eye-pieces to be made in accordance. A gain in field in some 

 instances results, but it is an open question whether this is of any 

 real advantage. The modern objective, the high-power one 

 especially, only gives sharp definition at one and the same time 

 over the central and intermediate zones of the field of view, the 

 outer one being always more or less fluffy ; hence the question 

 arises as to what real use is the enlargement of the field of view 

 under such circumstances. There are microscopists I have met 

 who have told me that they consider the modern objective is really 



