May 19, 192 1 J 



NATURE 



361 



that the content of the frame is the content of the 

 fabric in absolute rest, or to make any other deduc- 

 tion he logically can. He does not pretend to explain 

 what energy is or what it may become if reduced to 

 absolute rest. He does not assert that there is no 

 absolute rest, but that it escapes his and all ex- 

 perience. John G. McHardy, 

 Commdr. R.N. (Retired). 

 16 Eburv Sti-eet, S.W.i, May 6. 



The Production of Metallic Zinc. 



In the issue of Nature for April 28 I observe under 

 "Notes " (p. 279) a reference to the small volume on 

 zinc recently issued by the Imperial Mineral Resources 

 Bureau. In this reference it is pointed out that the 

 figures relative to the production of metallic zinc in the 

 United Kingdom for 19 13 do not harmonise mth ttie 

 figures of production and imports of zinc-ore. Natur- 

 ally so, for there are other factors involved in the 

 production of metallic zinc in any given year. The 

 output of metallic zinc is not necessarily derived 

 entirely from the ores produced at home or imported 

 in that particular year; the part played by 

 "secondary " production — that is, metal obtained from 

 hard zinc— is of importance in this connection. 



As regards the use of the expression "long ton," to 

 which the writer of the note objects, preferring the 

 words "statute ton," it has been made abundantly 

 clear in the prefaces to the Bureau's publications that 

 *'the weights are expressed in long tons— that is to 

 say, the British statute ton of 2240 lb." The ton 

 of 2240 lb., though the "statute" ton in the United 

 Kingdom, is not necessarily the "statute" ton in 

 other countries. The expression "long ton" has not 

 only the advantage of conciseness, but it is also well 

 understood tTiroughgut the mining and metallurgical 

 world. 



R. A. S. Redmayne, 

 Chairman of the Imperial Mineral 

 Resources Bureau. 



2 Queen Anne's Gate Buildings, 

 Westminster, London, S.W. i. 

 May 4. 



The Theory of Vi ion. 



Prof. Joly's papers on vision are very interesting. 

 He adopts the visual purple as the visual substance, 

 but there is no evidence that the rods are percipient 

 elements. The view that they are percipient elements 

 is based on errors, as, for instance, that certain 

 animals — the tortoise is the most quoted— possess pnly 

 cones ; that the periphery of the retina is colour-blind ; 

 and that the Purkinje phenomenon is not found with 

 the fovea. The tortoise has the rods and cones as 

 definitely marked and distinct from each other as in 

 man. Has anv reader seen a retina in which there 



are only rods or only cones in any animal? The 

 periphery of the retina is not colour-blind. Red of 

 sufficient luminosity can be seen to the extreme 

 peripherj'. The Purkinje phenomenon is found with 

 the fovea, and is a photochemical phenomenon. It is 

 very improbable that the rods are percipient elements. 

 An elaborate nervous mechanism is required to regu- 

 late the sensitiveness of the photochemical film, and 

 this appears to be the function of the rods. 



The stimulus in vision is undoubtedly liquid, as 

 shown by the movement of positive after-images. 



The decomposition of the visual purple stimulates 

 the ends of the cones. The ends of the cones consist 

 of a series of discs varying in diameter. 



F. W. Edridge-Green. 



May 7. 



A New British Land Planarian. 



Mr. Morison's discovery of the interesting 

 planarian worm {Rhynchodemus Scharffi) in a garden 

 at Chiswick, as described in Prof. Dendy's letter in 

 Nature of May 5 (p. 298), shows that this species has 

 a wider range than was at first anticipated. As- Prof. 

 Dendy states, it was first discovered in a Dublin 

 garden in 1894, but since that date it has turned up in 

 the Royal Botanic Gardens at Glasnevin, Dublin. I 

 thought it had probably been introduced into both 

 localities, but that nevertheless it was indigenous to 

 Ireland. 



It seemed to me most likely to have been brought 

 from the country with a load of turf. This view was 

 confirmed when, in April, 1901, I found several speci- 

 mens of this planarian worm in the open country 

 under a fallen tree-trunk near Ballymote, Co. Sligo 

 (see Irish Naturalist, vol. x., 1901, p. 133).. , 



R. FT Scharff. 



National Museum, Dublin, May 12. ;,,,,;... 



Sir Richard Redmayne puts forward two explana- 

 tions to account for the discrepancy in the statistics 

 published by the Im{>erial Mineral Resources Bureau. 

 The first of these, namely, that stocks of ore may be 

 carried over from year to year, is, in view of the rela- 

 tively small differences from year to year, inadequate 

 to account for the great discrepancy noted. Tht 

 second is, in fact, the true explanation. Secondary 

 zinc accounts for about one-half of the so-called zinc 

 output of the country, and thus seriously affects the 

 statistics. The Writer of the Note. 



NO. 2690, VOL. 107] 



Cutting Sections of Cotton Hairs. 



Ix our laboratory we have now, for some months, 

 utilised Mr. H. J. Denham's plan for celloidin-paraflfin 

 embedding of the cotton hairs, on the lines of 

 Kultschitzky's and other processes (Wordeij^ "The 

 Nitro-cellulose Industry," p. 805), described in Nature 

 of Mav 5, p. 299, which Mr. Denham kindly com- 

 municated to us when he first suggested it, and we 

 have found it most satisfactory. We immerse the 

 hairs in dilute celloidin, which is then boiled down 

 to a syrup (Gilson's process); the hairs are next 

 transferred to paraffin-chloroform, and thence to 60° C. 

 paraffin (Ide's process) ; this makes a very rapid 

 technique, cut sections being available within two 

 hours. We have also tried the method of Willows and 

 Alexander, but find it cytologically inferior to this 

 celloidin-paraffin technique, which gives us excellent 

 sections at 2-5/x setting on a Leitz sliding microtome, 

 with accidental sections even thinner. 



W. Lawrence Balls. 

 H. A. Hancock. 

 Experimental Department, 



The Fine Cotton Spinners' and 



Doublers' Association, Ltd., 



Manchester, May 13. 



British Scientific Instruments. 



In the review of the " Dictionary of British Scientific 

 Instruments " published in Nature of May 12, p. 324, 

 it is stated that the British Optical Instrument Manu- 



