December 14, 1911] 



NATURE 



213 



pJosions are actuall}' dust explosions was first stated in 

 England by Mr. Watson Smith, editor of the Journal of 

 the Society of Chemical Industry, in a letter which 

 appeared in The Glasgow Herald on July 12, 1872, immedi- 

 ately after the Tradeston disaster. The priority of Mr. 

 Watson Smith was recognised at the time by the Royal 

 Society of Edinburgh, and later (in 1882) by Sir Frederick 

 Abel in a lecture at the Royal Institution. 



It is interesting to know that nearly six years ago Mr. 

 Watson Smith read a paper at Liverpool (the scene of the 

 latest dust explosion) in which stress was laid on the fact 

 that any kind of carbonaceous dust might, under certain 

 conditions, become a source of danger (see Journ. Soc. 

 Chem. Ind., January 31, 1906). 



Albert Shonk. 



10 Dartmouth Road, Hendon, December's. 



I MUCH regret that I had entirely forgotten the fact, 

 stated in Mr. Shonk 's letter, that Mr. Watson Smith had 

 attributed the disaster at the Tradeston Flour Mills to an 

 explosion of dust in a letter to The Glasgow Herald, pub- 

 lished before the report of Profs. Rankine and Macadam 

 appeared, or I would certainly have mentioned it in the 

 article referred to. W. G. 



The Feeding Habits of Crepidula. 



W'lTH reference to the note on Crepidula in N-fiTURE of 

 December 7 (No. 2197, p. 187) it may be of interest to 

 your readers to kmw that during some recent researches 

 on this animal I have been able to confirm the necessity 

 for investigating how far the presence of the slipper-limpet 

 (Crepidula fortiicata) is a menace to successful oyster- 

 culture on the Kent and Essex coasts. It has been believed 

 by various naturalists that Crepidula takes the same kind 

 of food as the oyster, but on this point there exists no 

 definite information. During an investigation of this 

 matter I discovered the manner in which the animal feeds, 

 from which there can be no doubt whatever as to the 

 nature of its food. The mode of feeding in Crepidula is 

 the same in principle as that of the oyster, that is, there 

 is an ingoing and an outgoing current of water kept up 

 in the mantle-cavity, while between the two currents the 

 gill acts as a strainer, retaining even very fine particles 

 of suspended matter, which eventually — by one of two ways 

 — reach the mouth.' 



Thus it is established beyond doubt that Crepidula feeds 

 on the same material as the oyster, that is, on the food- 

 material found on or floating near the sea-bottom, and the 

 danger apprehended from this intruder is confirmed : 

 Crepidula is competing successfully with the oyster for food 

 and space. Whether there is enough food and space for 

 both Crepidula and oysters is another matter which must 

 be determined by local researches. Thus the problems for 

 the Kent and Essex oyster-farmers are to keep up the food 

 supply of oysters and to reduce the numbers of Crepidula 

 and the many other animals which take the same food as 

 oysters. J. H. Orton. 



Marine Biological Laboratory, Plymouth, 

 December 10. 





1i 



Tadpole of Frog. 



.At the beginning of .\pril last I collected some frog ova 

 for the purpose of making oijservations on development. 

 Tadpoles appeared about .\pril 9, and from time to time 

 from that date until July 17, when young frogs were 

 df^veloped, I took batches away for preservation and section- 

 iif. On July 17 only one tadpole was left of the original 



ick, and that one, though in water out of doors and 

 uith a supply of waterweed, has not developed farther, but 

 is a tadople still and is still alive. Some years ago I had 

 a similar case with a frog tadpole. Can any of your 

 If aders suggest the reason of this phenomenon ? 



T. Pr.owM.w. 



Nystuen, Bycullah Park, Enfield, December 8. 



' It isprcposefl to publish a 'full account of how Crepidula feeds in the 

 \t number of the Journ.il of the Marine Riolo(;ical Association.' 



NO, 2198, VOL. 88] 



MICROKINEMATOGRAPHY. 



YA/'ITHIX the last few months we have been shown 

 ** a new appplication of the kuiematograph, 

 which indicates yet another stage of technical attain- 

 ment, and another field in which it may supplement 

 our knowledge. Its range has been extended to the 

 representation of objects as seen through high powers 

 of the microscope. Apart from any positive increase 

 to knowledge which may be obtained by its means, 

 this is a technical achievement of a very high order. 

 In the usual microscopic preparation it is impossible 

 to obtain a high degree of illumination, and the 

 greater the magnification the less the illumination 

 becomes. It is only by artificially increasing the con- 

 trast by means of stains and so forth that we can 

 obtain a clear differentiation of even a motionless 

 object. To take in one minute some thousands of 

 successive photographs of a living, unstained object, 

 magnified si.x hundred or a thousand times, an object, 

 moreover, which is moving rapidly, and therefore 

 continually altering its focal plane, is a task which 

 might easily seem impossible. 



jNI. Comandon, however, has succeeded in this ex- 

 tremely difficult problem. The illumination-difficulty 

 he avoided by using what is known as the ultra- 

 microscope or dark-ground illumination, in which the 

 object is seen against a black background, being lit 

 itself by rays of light striking it from the side, and 

 thence deflected upwards towards the lens of the 

 microscope. This method gives an extremely brilliant 

 contrast-illumination of the outlines of the object 

 against a black ground and makes it possible to take 

 on a properly sensitised film photographs of exceed- 

 ingly short exposure. The resulting picture naturally 

 shows comparatively little of the internal structure of 

 the object under e.xamination ; the bulk of the rays of 

 light are deflected from its surface. But it is sur- 

 prising how much does appear. The nucleus of a 

 cell, for example, is frequently quite distinct, and some 

 structures, such as the kineto-nucleus of a trypano- 

 some can sometimes be seen perfectly clearly and be 

 followed as the organism moves from place to place. 

 A large number of films prepared under the direction 

 of M. Comandon has been exhibited during the pre- 

 sent year by Messrs. Pathe Fr6res, and the realism 

 and vitality of these kinematograph pictures can 

 scarcely be imagined by anyone who has not seen them 

 thrown on the screen. 



An interesting film is one which displays the blood 

 actually circulating in the vessels of the living body. 

 The preparation, which is from the tail of the tad- 

 pole, shows a number of tiny blood-vessels, which 

 measure about one-hundredth part of a millimetre in 

 diameter. Crowded together in the larger of these, 

 the individual corpuscles of the blood can be seen to 

 pass out one by one into minute branches, for which 

 they seem almost too large, and within which they 

 make their way here and there through the surround- 

 ing tissue, not apparently without occasional difficulty. 

 Even in the larger vessels, along which the bulk of 

 the corpuscles are hurrying, tlie rate of progress varies 

 considerably, and the' direction may actually be re- 

 versed for a time and the blood apparently flow back- 

 wards. The coloured corpuscles of the blood, from 

 which it derives its red tint, have, of course, no in- 

 dependent motion of their own, and are simply carried 

 along bv the stream in which they are suspended. 

 But the colourless cells or leucocytes have such in- 

 dependent motion, and in another film we are shown 

 a white cell gradually altering its shape, throwing 

 out a long filament into which the rest of the corpuscle 

 slowly flows, imtil the whole eel! has altered its posi- 



