34 president's address. 



billing glass, mounting material, and immersion fluid, all 

 having alike a refractive index equal to this aperture. 



Such a medium had been found, both for mounting the 

 objects and immersing, both the condenser with the mounting 

 slip and the front of the lens with the cover. But the mount- 

 ing medium has proved valueless because transient. All the 

 slides I have received from Germany have perished ; none has 

 lasted longer than five months. This is a block to a persistent 

 attempt to discover the real merits of the object-glass. It is 

 only by comparison under varying circumstances of a well- 

 known object that reliable conclusions can be drawn. 



If, therefore, after laborious work such a slide has been 

 examined and its special points "logged" and carefully noted, 

 it is more than discouraging to find it, as a mounting, wholly 

 destroyed by a kind of disintegration in a few months. 

 But even such a slide could only be a mount of silicious or 

 calcareous bodies, such as diatoms, spicules, foraminifera, and 

 so forth. Animal or vegetable tissues, or minute organic forms, 

 as I pointed out, must be irretrievably destroyed by the only 

 mounting media which could be employed. 



I expressed a hope that ultimately some medium, tolerant, 

 at least, of dead organic forms and animal and vegetable 

 tissues, might be discovered. It is not to-night, however, my 

 good fortune to report its discovery. But, happily, we are 

 never at a standstill. For some time we have been striving to 

 obtain a thoroughly monochromatic light for microscopical 

 purposes. With much confidence ammonia- sulphate of copper 

 solution was used, and various batches of blue glass, made both 

 in England and on the Continent. But when critically examined 

 they were found not to be monochromatic at all. We all 

 know that Hartnack constructed an arrangement of prisms, 

 afterwards made by Zeiss, for screwing on to the under side of the 

 stage, and throwing a not very widely dispersed spectrum on to 

 the image on the stage. 



The defect of this was its feebleness. It could not be used 

 with a condenser; in fact, was made before the Continental 

 makers had yet perceived that the condenser was a vital and 

 practical part of the optical action of a microscope. 



Clearly, therefore, no critical image with a lens of large N.A. 

 and considerable power could be produced by this means ; 



