ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 831 



but it is made even more surprising when we come upon tlie statement 

 lower down in the paper, " I have never seen a slide of diatoms mounted 

 " in biniodide of mercury and iodide of potassium," so that the cannot-be- 

 too-emphatic condemnation of the medium with which the author began 

 was not founded on any practical exjierience whatever. 



The climax, however, is not yet reached, for in a footnote the author, it 

 will be seen, states that he has now learnt that the diatoms will not fall to 

 the bottom of the cell, as ho had asserted, but will float and press upwards 

 against the cover-glass ! 



The following is the paragraph : — 



" Fluids. —Although certain of these media, such as biniodide of 

 mercury with iodide of potassium, as well as oil of cassia, can be ob- 

 tained with fairly high refractive indices, yet I cannot too emphatically 

 condemn them for use with the higher powers of the Microscope, simply 

 from the fact that the diatoms will not remain on the cover-glass, but 

 must necessarily fall to the bottom of the cell, which consequently 

 must be very shallow, otherwise the diatoms will be beyond the focus 

 of the objective. With shallow cells in fluid mounts the diatoms can 

 easily get crushed on cleaning the cover-glasses. If it were not for 

 these fatal objections, I should be disposed to regard oil of cassia very 

 favourably as a mounting medium, as these essential oils give great 

 brilliancy ; but whether they can be effectually sealed for a permanency 

 I cannot say. I once mounted a slide in oil of cloves, and it remained 

 perfect for some considerable time, but eventually a bubble made its 

 appearance. I have never seen a slide of diatoms mounted in biniodide 

 of mercury and iodide of potassium, and am inclined to think that this 

 medium is very little used. 



[Since writing the above I have learnt, with respect to the solution 

 of biniodide of mercury and iodide of potassium, that the medium is 

 of such high specific gravity — viz. 3 • 02 — that any diatoms which may 

 chance to become detached will float in the fluid and press upwards 

 against the covering- glass, instead of falling to the bottom of the cell."] 

 The paragraph headed " Canada Balsam " is, however, still more won- 

 derful than the preceding, as the author makes this statement : — " The only 

 "objection, to my mind, against this medium is that its refractive index 

 " is not sufficiently high for the new immersion lenses " ! Let us put the 

 refractive index of Canada balsam at its lowest limit and call it 1*52, 

 where are these new immersion lenses which, according to the author, 

 have a higher " refractive index " ? The simple explanation no doubt is that 

 the author was quite unaware of the principle on which the use of media 

 of high refractive index depends, but that does not make it any the less 

 lamentable that such matter should have been presented in a scientific paper 

 to a Microscopical Society at the present day. 



''A QuEKETT Club-Man." — My Microscope and some Objects from my Cabinet. A 



simple introduction to the study of the " infinitely little." 



78 pp., 5 figs., 8vo, London, 1887. 

 American Society of Microscopists — Pittsburgh Meeting. 



Amer. Hon. Micr. Journ., VIII. (1887) pp. 156-7. 

 Microscope, VII. (1887) pp. 248-50, 269-74. 

 Dallinger, W. H. — The Marvels of Microscopy. 



[Presidential Address to Devonshire Association for the Advancement of Science, 

 Literature, and Art.] 



Western Daily Mercury, 17th July, 1887. 

 May all, J., jun. — Conferences sur le Microscope. (Lectures on the Microscope.) 

 {Gontd.) 



\_Transl. of tlie Cantor Lectures.] 

 Journ. de Microjr., XL (1887) pp. 240-6 (1 fig.), 269-75 (2 figs.), 335-41 (9 figs.). 



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