25.2" 



N<ATURE 



[May 30, 19 1 8 



is fitted to compensate for local errors in each lead 

 screw. Each tool, when sold, is accompanied: by a 

 certificate from the National Physical Laboratory as to 

 its performance, and. the certificate gives the errors: 

 found in a screw 8 in. long actually cut in the machine. 

 The design of the machine, so far as can be deter- 

 mined from the drawings and photographs given in 

 the article, is excellent, and the lathe should f6rm a 

 valuable addition to the eqviipment of gauge shops. 



The May issue of the Transactions of the Optical 

 Society is devoted almost entirel}' to papers on the 

 methods of design of telescopic objectives. Mr. P. F. 

 Everitt sets out clearly in order of importance the six 

 conditions which it is desirable that an objective, 

 so far as is practicable, should fulfil. He then shows 

 how, by the help of tables such, as those of Smith and 

 Cheshire, the approxirhate radii of the surfaces of 

 the objective are found, and corrected by tracing 

 the paths of an axial and an edge ray through the 

 system. Mr. T. Smith gives an account of the 

 methods in use at the National Physical Laboratory 

 which have furnished the tables just mentioned, and 

 Mr. S. D. Chalmers gives an alternative method of 

 making the calculation. In the discussion of the 

 three papers Prof. Cheshire emphasised the import- 

 ance of accurate computation of the properties of an 

 objective before the manufacturer put tool to glass. 

 We cannot in modern times wait for a sample to 

 be made and tested before producing instruments in 

 quantity. Mr. Conrady and Mr. Hasselkus con-^ 

 tended that an objective should be designed to com- 

 pensate the errors of the common eye-pieces, while 

 Mr. Everitt declined to saddle the objective with this 

 task. 



Mr. C. Turnbull read a paper to tbe Institution 

 of Electrical Engineers on May g in which he urged 

 the necessity of having a "national proving house" 

 for testing British engineering apparatus and 

 materials. Although most of tire speakers agreed 

 with Mr. Turnbull, no one advanced any real proof 

 that there was any urgent necessity for a commercial 

 laboratory of this kind. We are not aware that there 

 is any appreciable quantity of inferior apparatus or 

 shoddy electrical materials in the market. The presi- 

 dent, Mr. C. H. Wordingham, in opening the dis- 

 cussion, gave a summary of a report of the com- 

 mittee of the council which had been considering 

 the subject. He began by saying that the proving 

 house would not enter into competition with existing 

 institutions, but it will be difficult to avoid doing so. 

 It will be remembered that when the National' 

 Physical Laboratory was started this consideration 

 caused considerable friction. As a proving house will 

 have to be largely, if not altogether, self-supporting, 

 little research work can be undertaken. Sir Richard 

 Glazebrook welcomed the suggestion that the proving 

 house should work in conjunction with the National 

 Physical Laboratory. The experience of the working 

 of the National Board of Fire Underwriters of the 

 U.S.A., which has what is practically an electrical 

 proving house, shows, however, that the main 

 problems it is forced to consider are political, com- 

 mercial, and international rather than scientific. 

 Hence it may be advisable to leave the problems of 

 a British national proving house to the engineers' and 

 manufacturers' associations, as they are free to deal 

 with such questions. Unless a much stronger case can 

 be mad6 out for it, the whole proposal will probably 

 fall through. 



In the Kjeldahl method _ for the estimation of 

 nitrogen in organic compounds the substance is 

 usually— in fact, almost invariably — digested with the 

 sulphuric acid until a clear, transparent liquid is ob- 

 tained. With some substances, e.g. indiarubber, a 



NO. 2535, VOL. lOl] 



very prolonged period of digestion is thereby rendered 

 necessary. Mr. Matthew Howie finds, howeveri 

 (Journal of the Society of Chemical Industry^ 

 March 30), that the whole of the nitrogen present in 

 rubber is converted into ammonia in less time than . 

 is required to effect the complete dissolution of the 

 substance. Using samples of plantation sheet and. of 

 Manihot rubber, it was found that 80 per cent, to 

 94 per cent, of the nitrogen was converted into am- 

 monia after one hour's digestion^ whilst three to four 

 hours' digestion gave as high a nitrogen value as- the 

 six hours necessary for complete clarification of the 

 solution. It is possible that in the case of other 

 highly resistant nitrogenous substances the Kjeldahl 

 estimation might be similarly shortened. 



OUR ASTRONOMICAL COLUMN, 



May Meteors. — Between May 17 and 24 meteors 

 have occasionally been abundant and given evidence of 

 several well-defined showers. Whether or not this period 

 is worthy of special note cannot be absolutely affirmed, 

 though the evidence strongly suggests that it needs 

 further investigation. This year some fine meteors- 

 were observed at Bristol on the mornings of May 18 

 and 19, and proved that several of the various systems 

 which mark this epoch returned with tolerable strength. 



In 1866, May 18, several of the assistants of the 

 Royal Observatory, Greenwich, remarked a striking 

 prevalence of bright meteors, and Mr. Denning found, 

 on projecting the roughly observed paths that the 

 radiant was placed at 247° + 32° near t, Herculis. 

 This shower was observed at Bristol in 1903, 191 1, 

 and a few other years, and from a general investiga- 

 tion of all the meteor tracks recorded at the latter 

 station since 1875 during the period May 17-24 the 

 following radiants seem well defined :— 



Many periods of the year appear to be more note- 

 worthy for the large number of streams visible than 

 ifor the special richness of one or two. 



Distance of the Pleiades. — Prof. W. H. Pickering 

 has made a further application of the statistical 

 method to the determination of the distance of the 

 Pleiades (Harvard Circular, No. 206). Absolute 

 magnitudes were calculated by Russell's formula 

 M = o-6 + 2i(T— 2), where M, is the absolute magni- 

 tude and T the type of spectrum, counting B as i, 

 A as 2, F as 3,.G as 4, and K as 5. The eighty-two 

 stars considered range in type from B5 to A9, and, 

 omitting the six brightest stars as being, possible 

 "super-giants," the average difference between ap- 

 parent and absolute magnitudes is 6-46. This corre- 

 sponds with a distance of 201 parsecs, or 656 light- 

 vears, the parallax being 00050" ±00008". It thus 

 appears that the Pleiades are about five times as 

 remote as the Hyades, while the distance between 

 the two farthest apart of the bright stars (63') is 

 12 light-vears. The brightness of- Alcyone is esti- 

 mated to be 2100 times that of the sun, while the 

 other five bright stars average about 800 times: as 

 bright as the sun. 



By the same method Prof. Pickering finds 301 light- 

 years, or a parallax of 00109" + 00026", for the Coma 

 Berenices cluster. 



TlHE Spectroscopic Binary 42 Capricorni. — . 

 The variable radial velocity of this star was shown in 



