THE president's ADDRESS. 301 



There is no necessity for working in the dark, although access 

 to the room of orclinaiy dayhght, as well as of unscreened lamp- 

 light, must be avoided. A good plan is to use a Cambridge 

 reading-lamp placed at some little distance from the observer, its 

 green-glass shade being so arranged as to prevent the access of 

 any but modified rays to the eye. It would scarcely be believed, 

 by those who have not tried it, how greatly the sensitiveness of 

 the retina is increased by this simple proceeding, and how much 

 more readily the most delicate details are perceived w^hen none 

 but rays thus modified obtain entrance to the work-room. 



The medium with wdiich the first of the experiments that I 

 shall mention were made is a ^-olution of biniodide of mercury 

 in excess of iodide of potassium. This solution is very readily 

 obtained, and, being an aqueous one, is easily retained within a 

 ring of dammar and gold size cement.* I am indebted to Pro- 

 fessor Fuller, of Surbiton, for a knowledge of the high qualities 

 of this mounting medium, as w^ell as for a considerable number of 

 well-mounted slides wdiich he has generously placed at my disposal. 

 Unlike many other highly refracting media, the biniodide is colour- 

 less, and therefore requires no special modification of illumination. 

 In common with some other mounting fluids, this medium seems 

 to be suitable for only certain tests. Used on SurireUa yemma 

 and on A. pelhccida, with suitable illumination and with direct 

 light, it brings out structure with a clearness and beauty wdiich 

 I do not remember to have seen surpassed. 



In mounting diatoms in this medium, however, care must be 

 taken to make the film of fluid in the cell as thin as possible, for 

 otherwise the diatoms, which are easily detached from the cover 

 on which they are, or should be mounted, change their position 

 rapidly whilst under observation in the microscope, and thus cause 

 considerable annoyance. This might, of course, be remedied if an 

 unobjectionable method could be devised of fixing the diatoms 

 to the cover glass. Heating the cover glass with the diatoms in 

 situ has been suggested, but this plan is certainly not always 

 successful ; and, as far as I know, there is no material, insoluble 

 in water, wdiich can be used for fixing the valves to the cover 

 glass, and which will not at the same time damage the object for 

 purposes of observation. 



* Made after tlie excellent formula of Mr, Eousselet— viz., two parts of 

 a satuisated solution of dammar in benzol, and one part of gold size. 

 JouRN. Q. M. C, Series II.— No. 46. 22 



