THE president's ADDRESS. 351 



ought to be strongly impressed on every beginner. If you show 

 a fairly large diatom such as Heliopelta or Aidacodiscus formosus 

 or Kittonii to one only acquainted with the estimation of depth 

 by means of the fine adjustment with a monocular, and if you 

 let him form his own conclusions as to the shape of the object, 

 with which he should not be previously acquainted, and after- 

 wards show him the same object as opaque, with a stereoscopic 

 binocular, he will receive a terrible shock as the truth dawns 

 upon him that, although he has been a microscopist for many 

 years, he has never rightly comprehended the true form of a 

 single object he has ever examined. 



If to-night there are here any beginners in microscopical 

 work, let me earnestly advise them to render any such shock 

 impossible by a timely study of opaque forms with a binocular ; 

 but if there are amongst us any advanced microscopists who 

 have not as yet prosecuted this form of research, let me urge 

 upon them to enter at once upon a similar course of study, and 

 so render the shock less than it would be if it were to come in 

 subsequent years. Leaving the mechanical, we will pass on to 

 the optical portion of the microscope ; here we find continuous 

 advances in that important branch, viz., cheap objectives. 



Although the aperture of the cheap oil ^^ remains much the 

 same, viz., 1-25 — 1-30 N.A., the corrections for both the 

 chromatic and spherical aberrations have undergone consider- 

 able alterations. The changes in the chromatic aberrations 

 will be the more noticeable, as those violent purples and reds 

 with which we are all acquainted are gone, and now are re- 

 placed by far paler tints. It is said that some of these lenses 

 have been corrected for photography ; this, in the fullest sense, 

 means that a photograph will be true to the visual focus with 

 an ordinary plate, when no screen is employed. I have not as 

 yet experimented with the most .recent objectives in this 

 direction, but the statement that those of a year or two back 

 T^ere so corrected cannot be maintained if other than the 

 narrowest angled cones of illumination were used. Nevertheless 

 we have all seen the fine results lately obtained by Mr. Smith 

 with cheap objectives on isochromatic plates, one taken with a 

 new lens by Messrs. Swift and Son being specially noticeable. 

 The improvements in the high powers, indicated above, have 

 also been carried out with marked success in the medium 



