September, 1911 



KNOWLEDGE. 



333 



in the same plate, we have the outside of P. 

 diifiulatiiiu, showing a different structure; and Mr. 

 Smith has abundant evidence of the existence of 

 what he has so long maintained. 



'■ By using the new lens of the great aperture of 

 1-63, Dr. Van Heurck has produced some remark- 

 able photo-micrographs, which rather confirm these 

 general inferences than present any new data of 

 knowledge concerning the diatoms.'" 



As it happens. Figure 6 of the plate of Dr. \'an 

 Heurck shows an isolated strip almost identical with 



that given here in Figure 18, even to the breaking 

 across in one particular spot. It should be interesting 

 to compare the two, not onl\- the strip, but also the 

 edges of the fractured valve bordering the blank 

 spaces on each side the strip. Figure 5, of the 

 same plate, exhibits the hexagons magnified 

 ten thousand diameters, which Sir (then Mr.) 

 I'rank Crisp, described, on its introduction to the 

 Royal Microscopical Societ}-, as a remarkable 

 photo-micrograph. It proved subsequently to 

 be onlv an enlargement from the original negative. 



Note.— In the first part of the atticle, on page 289, Figures 1 and 2 were inadvertently interchanged, and on page 291, 



■■720" should read 1720. 



CORRESPONDENCE. 



OBSERVATION OF URANUS AND VESTA. 

 To the Editors of '" Knowledge." 



Sirs, — Mr. Leonard speaks of Uranus being probably 

 visible to the naked eye, and in this connection I may say that 

 the planet is often distinctly visible without telescopic aid. 

 Many years ago, while pursuing meteoric observation, it was 

 my habit to obtain nightly glimpses of Uranus and I found it 

 quite possible to trace, even with unaided vision, the displace- 

 ment in the position of the object relatively to small stars 

 near. In fact I considered it an attainable feat for the 

 ancients to have discovered the planet with the unassisted eye 

 had they watched the zodiacal stars with great diligence and 

 compared exact positions. Indeed Vesta, as well as Uranus, 

 might have been recognised in this manner. 



\V. F. DENNING. 



THE DISTRIBUTION OF THE NIGHTINGALE. 

 To the Editors of "Knowledge." 



Sirs, — As in the August " Knowledge " you invite 

 observations on the distribution of the Nightingale, I take 

 the liberty of sending the following : — 



In the map given in "British Birds," it is shown as 

 numerous, or fairly so. over the greater portion of this 

 county. Now in the Devizes and Pewsey Vale district it is 

 extremely rare. My father has been in this part of the 

 county for thirty- five years, and the first instance of its 

 occurring here he has heard of was on May 3rd, 1910, when 

 one came to our garden and stayed for two d.ays and a night 

 (i.e., it was on migration). 



Round Salisbury and in South Wilts generally it is a 



common bird. 



G. B. HONV. 



THE TRUE STRUCTURE OF THE DIATOM VALVE. 

 To tlie Editors of " Knowledge." 



Sirs, — With reference to Mr. T. F. Smith's article on the 

 above, it would seem a word or two is needed. At the outset 

 let me heartily congratulate him upon his most excellent 

 photographs of the valve. From one or other of them 

 absolutely the whole structure is laid b.are. but what at the 

 same time is truly astounding is the fact, that having this 

 actuallv before him he needs misinterpret it and head off upon 

 a " new structure discovery " ending up with " chains of 

 fibrils formed of short bars of silex arranged lengthwise 

 which run in pairs parallel, and each pair having larger 

 and narrower interspaces between, in regular succession, and 

 so on." All of which is totally beside the mark and quite 

 misleading to amateur microscopists. 



These "chains of fibrils" in Pleurosigina, as well as 



Mr. Smith's "pins" in Podtira scales, are as spurious to the 

 true structure as the Man in the Moon. One is aware that 

 kaleidoscopically they may be observed and even photo- 

 graphed, but taking his own example of sheets of transparent 

 paper with markings thereon placed behind one another, 

 although he has separated the sheets, in one photograph or 

 the other, he ends with placing three together (at least) under 

 a peculiar illumination and gives this as the true structure. 

 It is disappointing that with such excellent photographs the 

 time and patience should be expended in finally misreading 

 their purport and sallying forth into print to further mislead 

 mayhap other amateur microscopists. As I have only seen 

 the first portion of Mr. Smith's article published, I am 

 anxious to see what further discoveries he has in store for us. 



F. J. W. PLASKITT. 



Sirs. — By your courtesy I have received a proof of Mr. 

 F. J. W. Plaskitt's letter printed above, which gives me the 

 opportunity of replying, without the delay of another month. 

 For his kind reference to the excellence of my photographs I 

 thank him, at the same time taking exception to his strictures 

 on some of my work. He says, for instance: These " chains 

 of fibrils" in Pleiirosignia, as well as Mr. Smith's "pins" in 

 Pod lira scales, are as spurious to the true structure as the 

 " Man in the Moon." By " spurious," I suppose he means due 

 to diffraction effects. If so, why do they leave off' in certain 

 places, while at the same time there is an extended structure 

 immediately underneath ? Not only that, the structure I 

 exhibit is irregular, while diffraction effects are not. DiftVaction 

 effects result also from a narrow cone of illumination, while I 

 always work with a wide one of strictly central light. 



Mr. Plaskitt says that in one or other of my photographs : 

 " absolutely the whole structure is laid bare " — then — " but 

 what at the same time is truly astonishing is the fact, that 

 having this actually before him he needs misinterpret it and 

 head oft' upon a new structure discovery " and so on. 

 Further ; " He ends with placing three together (at least) " — 

 does Mr. Plaskitt mean three layers of structure, or three 

 photographs ? — " under a peculiar illumination and gives this 

 as the true structure." 



Unfortunately he does not give the numbers of the figures to 

 be praised, and otherwise, for guidance, leaving me quite in 

 the dark as to the work for which I am to be blessed and for 

 which to be banned. As the other part of my article appears 

 in this number of " Knowledge," I suppose he will now 

 have further exceptions to take; will Mr. Plaskitt kindly 

 supply the references I ask for, when I shall be pleased to 

 do my best towards meeting his points. I have never run 

 away from a discussion yet, but I naturally want to know 

 what I am fighting. 



T. F. SMITH. 



