100 NOTES. 



discovery that by drawing out the draw-tube an increase of magni- 

 fying power could be obtained, and who was of opinion that with 

 an unlimited length of tube an unlimited power could be got ; 

 and that by using an electric light and a ^-inch objective, he 

 would, by going to the top of the house and looking down through 

 an eyepiece, be able to obtain enormous magnifying power ! " — 

 Journ. R.M.S., 1895, p. 256. 



In a recent number of The Diatomiste (11., pp. 39, 40), Prof. 

 J. Brun claims to have settled the question that the pearls of 

 diatoms are cavities, and not protuberances, in the following way : 

 If a particle of a transparent body immersed in a liquid is first 

 exactly focussed under the microscope and the tube then raised, 

 the object will appear to have a bright centre if its index of refrac- 

 tion is higher than that of the liquid ; a dark centre if the index 

 is lower. If the tube is lowered, the results are the opposite. 



If a diatom with large areas — such as a Coscinodisais — is 

 immersed in styrax or monobromide of naphthalin, and examined 

 in this way, the pentagonal siliceous network is shown to have a 

 lower index of refraction than the styrax, while the interior of the 

 pentagon has not, showing that it must be a cavity, with the same 

 index as that of the surrounding medium. If immersed in water, 

 the inverse phenomenon is observed. -^<?z^r«. R.M.S., iSpSi P- 213. 



Mr. G. C. Whipple thus sums up an interesting paper, which he 

 has contributed to the Technology Quarterly (vii., 1894, pp. 214 — 

 231), anent the growth of diatoms in surface waters : he considers 

 " That the growth of diatoms in ponds is directly connected with 

 the phenomenon of stagnation ; that their development does not 

 occur when the lower strata of water are quiescent, on account of 

 greater density, but rather during those periods of the year when 

 the water is in circulation from top to bottom. 



That diatoms flourish best in ponds having muddy bottoms. 



That in deep ponds there are two well defined periods of 

 growth : one in the spring and one in the fall ; that in shallow 

 ponds there is usually a spring growth, but no regular fall growth, 

 and that other growths may occur at irregular intervals, as the 

 wind happens to stir up the water. 



That the two most important conditions for the growth of 

 diatoms are a sufficient supply of nitrates and a free circulation of 

 air, and that both these conditions are found at those periods of 

 the year when the water is in circulation. 



That while temperature has possibly a slight influence on the 

 growth of diatoms, it is of so little importance that it does not 

 affect their seasonal distribution. 



That the increase of diatoms takes place substantially in 



