ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 827 



Cox, C. F.— Remarks on Photomicrography. 



rPrincipallv as to letting the negatives alone after they are taken.] 



•- Jonrn. New York. Mwr. Soc, 1887, pp. 18-9. 



H., G. M.— A simple Photographic and Photomicrographic Apparatus. 



Eyi'll. Mcch., XLV. (1887) p. 503 (12 figs.), from Scientific American. 

 King, Y. M.— The Photomicrography of Histological Subjects. 



New York Med. Journ., II. (1887) pp. 7-11. 



Photo-Microscopy. I., II. Ckarterhonse Phot. Art. .Joum., I. (1887) pp. 2-4. 



Rovx. E.— La Photographic appliquee a I'etude des microbes. (Pliotography applied 



to the study of microbes.) Ann. de I'Institat Pasteur, 1887, pp. 209-25. 



(5) Microscopical Optics and Manipulation. 



Limit of Visibility. — In his Presidential Address at the Manchester 

 Meeting of the British Association, Sir H. Roscoe appears to have fallen 

 into a not unimportant mistake with regard to the smallest dimensions 

 which can be distinguished by the Microscope. 



In dealing with atoms he said : — 



"Next let us ask what light the research of the last fifty years has 

 thrown on the Daltonian atoms : first, as regards their size ; secondly, in 

 respect to their indivisibility and mutual relationships ; and, thirdly, as 

 regards their motions. 



As regards the size and shape of the atoms, Dalton ofi"ered no opinion, 

 for he had no experimental grounds on which to form it, believing that 

 they were inconceivably small and altogether beyond the grasp of our 

 senses aided by the most powerful apjiliances of art. . . . 



But modern research has accomplished, as regards the size of the atom, 

 at any rate to a certain extent, what Dalton regarded as impossible. Thus, 

 in 1865, Loschmidt, of Vienna, came to the conclusion that the diameter 

 of an atom of oxygen or nitrogen was 1/10,000,000 part of a centimetre. 

 With the highest hnoion magnifying power we can distinguish the 1/40,000 

 part of a centimetre; if now we imagine a cubic box each of whose sides 

 has the above length, such a box when filled with air will contain from 

 60 to 100 millions of atoms of oxygen and nitrogen. A few years later 

 William Thomson extended the methods of atomic measurement, and came 

 to the conclusion that the distance between the centres of contiguous 

 molecules is less than 1/5,000,000 and greater than 1/1000,000,000 of a 

 centimetre ; or, to put it in language more suited to the ordinary mind, 

 Thomson asks us to imagine a drop of water magnified up to the size of 

 the earth, and tells us that the coarseness of the graining of such a mass 

 would be something between a heap of small shot and a heap of cricket- 

 balls. Or, again, to take Clifford's illustration, you know that our best 

 Microscopes magnify from 6000-8000 times ; a Microscope which would 

 magnify that result as much again would show the molecular structure 

 of water. Or again, to put it in another form, if we suppose that the 

 minutest organism we can now see were provided with equally powerful 

 Microscopes, these beings would be able to see the atoms."* 



Microscopists will readily recognize that the 1/40,000 of a centimetre 

 — which is approximately 1/100,000 of an inch — is vastly too low a figure, 

 which should be at least 5 times smaller. Dr. Eoyston-Pigott claims to 

 have seen the 1/1,000,000 of an inch, but, whether he has or not, it is 

 certain that the 1/500,000 of an inch has been distinctly recognized. 

 Moreover, Sir Henry himself, as will be seen, states that a power of 8000 

 times is attainable "with our best Microscopes"; multiply 1/100,000 in. 



* Cf. Nature, xxxvi. (1887; p. 417. 



