THE president's ADDRESS. 305 



glass in the ordinary way, a small portion of the mixture is 

 placed between the cover' glass and the slide and gently fused 

 until a thin film of it unites the two surfaces. Only sufficient of 

 the medium should be used to fill about two-thirds of the area 

 of the cover glass, so that space may be left for a protecting 

 ring outside the medium. 



When the medium has set — ^which it does immediately on 

 cooling — protection from the air is necessary in order to prevent 

 decomposition of the combined salts. This is most readily effected 

 by allowing; a small portion of solid paraffin to run between 

 the cover glass and the slide by capillary attraction, so as com- 

 pletely to surround the deep yellow medium. If, after this, the 

 cover glass be encircled with a ring of Hollis's liquid glue, by 

 the aid of a turntable, the mount will be complete; and, judging 

 from the present condition of a specimen in my possession which 

 was mounted ten years ago by Mr. Morland, this medium possesses 

 high qualities of permanence. 



I am indebted to Professer Fuller for a considerable variety of 

 diatoms mounted in Piperine and Antimony Bromide. All the 

 finely lined species -such as A. jyeUitcida, ^'^itzschia sigmoidea, etc., 

 — are beautifully shown in this medium, and so are Surirella 

 gemma and the pleurosigmas ; but it does not answer equally well 

 for such diatoms as Coscinodiscus, or for any of the other coarse 

 circular forms that I have examined, up to the present date. 



A short time before the delivery of my former presidential 

 address I had, for the first time, the privilege of examining a 

 new achromatic condenser on the homogeneous immersion princi- 

 ple, so arranged as to transmit an aplanatic cone of at least 1 "3 

 numerical aperture. Since that time I have had an opportunity 

 of examining other similar condensers, some of them made by 

 Messrs. Watson, and more recently one or two by Messrs. R. and 

 J. Beck, the latter being simply and conveniently mounted, and 

 being procurable at a very reasonable price. For ordinary pur- 

 poses I do not believe that these instruments will ever supersede 

 the carefully constructed dry condensers of N.A. "90 or '95, which 

 have hitherto been in use almost exclusivel}'. Oil immersion 

 condensers require, for their successful employment, not only the 

 most minute exactness in the measurement of the glass mounting 

 slips, which, with the cover glass and medium together, must be 

 just thick enough, and not too thick, to occupy the space between 



