The President's Address. By Prof. P. Martin Dunean. 175 



There certainly is a cause of error latent in many mirrors, 

 and many Microscopes of the small type have their reflectors so 

 made as to reflect light quite as much from the brass edge as from 

 the glass. Carefully examined they are found to be badly silvered, 

 and the result is unequal light. There is no doubt that many of 

 our best practical microscopists use direct light whenever they can ; 

 and it is equally true that mirrors and their reflected light are 

 often very difficult to manage. Perhaps the re-introduction of plane 

 silver mirrors vyill relieve workers from some trouble. 



It is almost a presumption to remind those whom I have the 

 honour to address that there is no gain by amplifying at the 

 expense of definition, and that the short eye-piece with its high 

 power searches out and makes the errors of the objective manifest. 

 But it may be asserted that comparatively few microscopists select 

 that eye-piece which produces the best results in combination with 

 a given objective. The medium eye-piece is almost invariably used. 



Now there is, as we hope to hear clearly demonstrated shortly 

 by Professor Abbe, a necessity for balancing the performance of the 

 Microscope between the objective and eye-piece, in a manner which 

 is not simply empirical. 



There ought to be (the length of the tube of the Microscope 

 being invariable) a positive relation between the performances of the 

 upper and lower system of lenses, and therefore between the 

 magnifying power of the eye-piece and the numerical aperture and 

 amplification of the objective. Experience proves that it is not the 

 longest eye-piece or a regulation medium one which will always 

 produce the best results with a given objective, and that it by no 

 means follows that the eye-pieces supplied with a Microscope will 

 suit the objectives which may be added from time to time, or even 

 those originally put up with it. 



Without carrying this subject further I might state that in my 

 own case a medium and a long eye-piece of excellent workmanship, 

 and which usually give excellent results, do not evolve the qualities 

 of a Zeiss objective (E) which is of a moderate numerical aperture, 

 so well as one of slightly higher power and shorter tube than the 

 medium one just noticed. But this eye-piece does not do the same 

 service when a Grundlach water-immersion (1-1 2th in. focus) is 

 used, and the longest eye-piece then answers, although the objective 

 was made for the length of tube generally used in this country. 



Circumstances have brought me in contact with cheap Micro- 

 scopes, and certainly whilst it may be said that some of the 

 objectives are fairly good, the eye-pieces are on the miserable 

 " par " with the rest of the apparatus. I cannot avoid believing 

 that during the next few years attention will be paid to increasing 

 the merits and adaptability of eye-pieces whatever may be their 

 special character. 



