April 4, 1918] 



NATURE 



93 



The lens is placed immediately behind the screen, and 

 •^'v- thin pencils of light which pass through the holes 

 received on a photographic plate placed at suitable 

 iuts between the lens and its focus, and beyond the 

 focus. From the subsequent measurements of the 

 positions of the spots of light on the plate curves show- 

 ing the variation of the effective focal length, the 

 spherical aberration and the coma for each of the four 

 Ivinds of light used are drawn. Seventeen sets of 

 curves for typical lenses are reproduced in the paper. 



The Institution of Electrical Engineers has issued in 

 pamphlet form the standard clauses for street lighting 

 specifications which are the outcome of the delibera- 

 tions of the Joint Committee consisting of delegates 

 of the Institution of Electrical Engineers, the Institu- 

 rion of Gas Engineers, the Institution of County and 

 Municipal Engineers, and the Illuminating Engineer- 

 ing Society, and appointed in 1910. The specification 

 prescribes the form of tender, particulars of lighting 

 units, and the general nature of the contract. It is 

 proposed to classify streets in five classes, having 

 respectively, a minimum illumination of 001, 0025,004, 

 006, and o-i foot-candle, the minimum being measured 

 with a suitable photometer in a horizontal plane 

 3 ft. 3 in. from the ground. A minority report ex- 

 presses the dissent of the council of the Institution 

 of Gas Engineers to the proposed basis of measure- 

 ment, and it is preferred that contracts for street light- i 

 ing should be based on the average candle-power of | 

 the light source ascertained at two or three prescribed 

 angles. The points at issue were dealt with in the dis- 

 cussion of a paper on this subject by Mr. A. P. Trotter 

 before the Illuminating Engineering Society in 1913, 

 but complete agreement Avas not attained. As the 

 matter has been under consideration for six years it 

 was decided to publish the clauses as they now stand, 

 accompanied by a minority report. It is hoped that 

 they will be found useful when methods of street 

 lighting are reviewed after the war. Copies can be 

 obtained from the secretary of the institution, price 3d., 

 post free 4^. 



The relation between temperature and the pressure 

 of a saturated vapour is of great theoretical and prac- 

 tical interest, and a very extensive literature already 

 exists on the theoretical side, special attention having 

 been given to the relationship in the case of water and 

 water vapour, and ice and vapour. So far as the 

 vapour pressure of ice is concerned, experimental deter- 

 minations have been carried out with considerable 

 accuracy in recent years by Scheel and Heuse and 

 others, but there has been a great need for a series of 

 exact determinations of the vapour pressure of ice at 

 low temperatures, in order partly to correct the values 

 obtained by Scheel and Heuse, and partly to 

 see whether the Nernst formula holds good down to 

 the lowest pressures. Such a series of determinations 

 has been carried out with the greatest care by Sophus 

 Weber, w^orking in the laboratory of Prof. Kammer- 

 lingh-Onnes at Leyden (Communications from the 

 Physical Laboratory of the University of Leyden, 

 No. 150). The method employed was the ordinary 

 static method in combination with the absolute mano- 

 meter and the hot-wire manometer of Knudsen. The 

 measurements extend over a range of temperature from 

 about -22° to -190° C, and the values have been 

 compared with the Nernst formula, 



log/= !ll^-i!Z + i75 log T-000210T + 6-5343. 



^ The concordance has been found to be particularly 



good. By the introduction of a quantum-formula due 



to PolHtzer, Nernst has made his equation more 



rational, but so far as agreement with experiment is 



NO. 2527, VOL. lOl] 



concerned, there appears to be little to distinguish 

 the two expressions. Incidentally, it may be said, the 

 experiments of Weber show that water vapour at a 

 temperature of —80° has a molecular weight of about 

 20, whereby partial association is indicated. 



Thb customary methods for the preparation of plant 

 nucleic acids are rather cumbersome and necessitate a 

 peptic digestion of the nucleoproteins extracted. Messrs. 

 G. Clarke and S. B. Schryver have succeeded in avoid- 

 ing the peptic digestion, and their method of procedure 

 is described in the Biochemical Journal for December. 

 In the preparation of nucleic acid from yeast, the latter, 

 after pressing, is treated with a large excess of 95 per 

 cent, alcohol for twenty-four hours, and then boiled 

 for two hours in the same solvent, whereby the pro- 

 tein-complex is rendered insoluble in sodium chloride 

 solution. The yeast is then filtered, pressed, dried at 

 37°, ground to a fine powder, and extracted for four 

 to five days with 10 per cent, sodium chloride solution 

 at 6o°-8o°. When the clear extract is treated with 

 hydrochloric acid a characteristic precipitate of nucleic 

 acid separates and settles to a hard cake at the bottom 

 of the vessel. After standing, this is washed with 

 50 per cent, alcohol until free from chlorine, left stand- 

 ing overnight in 95 per cent, alcohol, and finally washed 

 with absolute alcohol and ether. The yield varied 

 from 14 to 1-6 per cent, of the dry alcohol-extracted 

 yeast. The crude nucleic acid was best purified by 

 dissolution in warm 10 per cent, sodium acetate solu- 

 tion, and reprecipitation with hydrochloric acid. 

 Nucleic acid can be prepared from wheat embryos in 

 a similar manner, but in this case it is found advan- 

 tageous to remove the starch by hydrolysis with taka- 

 diastase before extracting with sodium chloride solu- 

 tion. 



OUR ASTRONOMICAL COLUMN. 



Infra-red Solar Spectrum. — By the use of plates 

 stained with dicyanin, Mr. W. F. Meggers, of the 

 Washington Bureau of Standards, has obtained an 

 excellent series of photographs of the solar spectrum 

 in the region from 6800 A to 9600 A {Astrophysical 

 Journal, vol. xlvii., p. i). These photographs thus 

 provide material for accurate determinations of wave- 

 lengths in continuation of the classic tables of Row- 

 land, which did not extend further than the approxi- 

 mate limit of the visible spectrum at 7300 A. Photo- 

 graphs in the same part or the spectrum of more than 

 forty of the chemical elements have also been taken» 

 and nearly 400 of the solar lines have been identified 

 with lines in the spectra of eighteen elements. Two 

 hundred lines are accounted for by iron, sixty-three by 

 nickel, twenty-seven by titanium, twenty-two by cobalt, 

 and smaller numbers by chromium, silicon, manganese, 

 calcium, and other elements. One thousand six hundred 

 lines remain for the present unidentified. In addition 

 to the well-known bands due to terrestrial oxygen, 

 there are others which appear to be due to water 

 vapour. The separation of the solar and telluric lines 

 has been undertaken at the Allegheny Observatory by 

 the solar rotation method. Publication of the wave- 

 lengths is postponed, but reproductions of the solar 

 photographs, with wave-length scales, are included in 

 the paper. 



Harvard College Observatory. — A recent report 

 of the committee appointed to visit and report upon 

 the Harvard College Obser\'atory refers chiefly to the 

 valuable services rendered by the director in promoting 

 co-operation among astronomers. It is now about 

 forty years since Prof. Pickering began to advocate 

 the advantages of united efforts in carrying out some 

 of the larger investigations in astronomy, and at the 

 present time a considerable amount of the work of the 



