524 



NATURE 



[Sept. 29, 1887 



with concave gratings. He'first tried a grating of 1 2 feet focal 

 length, and the results were not as good as he tho«ght they 

 should be, so he then constructed a grating of 21 feet focal 

 length, and the results he would be able to submit to the Section 

 in his photographs. Having made the negatives, the next thing 

 was to place the scale upon them. He first tried Angstrom's 

 numbers, but they would not match. He had therefore to 

 determine the relative wave-lengths, and this he did by using 

 overlapping spectra and micrometer measurements. As the 

 spectrum was normal, all that was necessary was to get the scale 

 to ajree at two points of the photograph, and then it would 

 agree at all. He had found it necessary to adopt a new scale. 

 He was now engaged in making measurements in the red end of 

 the spectrum in order to complete his work. This he is doing 

 by the eye, and not by photography, as in this part of the 

 spectrum photographs do not show so much as the eye. In 

 laying the maps before the Section Prof. Rowland said that it 

 would be seen how crowded the lines were in the ultra-violet 

 region. He believed that on this account it would be almost 

 impossible to determine the metals to which they belonged. 



Capt. Abney thought it a serious thing to change the standard 

 of wave-length, and suggested that a Committee of the Associa- 

 tion should be appointed to confer with an American Committee 

 on the subject. 



Prof. Rowland said that Angstrom's numbers do not agree 

 among themselves, and therefore he could not fit a scale to his 

 map in any way until he had made a new determination. 



Prof Young, of America, agreed with Prof. Rowland in 

 thinking that Angstrom's numbers were not consistent. 



Mr. R. T. Glazebrook, F.R. S., exhibited negatives of photo- 

 graphs of the solar spectroscope taken by Mr. G. Higgs. The 

 spectroscope was one constructed by Mr. Higgs himself 

 Twenty-one lines can be counted on the negatives between Hj 

 and Hg. There is a length of about 4 inches between G and H, 

 and from 900 to 1000 lines can be counted between them. 



Recent Determinations of Absolute Wave-length, by Mr. L. 

 Bell, of Baltimore. — Some two or three years before Angstrom's 

 death he became aware that there was an error in the standard 

 metre used in his researches. Nothing in the way of correction was 

 done, however, until some three years ago, when Thalen 

 obtained a more accurate value for the metre and applied the 

 appropriate correction to Angstrom's wave-lengths. This 

 amounted to i part in 8500. 



Some few years ago a very careful determination was made by 

 Mr. C. S. Pierce, and it was with the view of confirming or cor- 

 recting his result that the writer began work. Pierce had found 

 an absolute value corresponding to the wave-length 5896 26 for 

 the less refrangible of the D lines, as the mean result from four 

 gratings. The writer using two of Prof Rowland's glass gratings, 

 obtained for the same quantity respectively 5893*95 and 5896 'ii. 

 The outstanding error must be ascribed to faults in the gratings. 

 Nearly all gratings are afflicted with variations in the distance 

 between the lines in various portions of the ruling. If the 

 irregularity is extensive, the grating is likely to show false lines 

 and give bad definition. 



If, however, the abnormal spacing — however great — is con- 

 fined to a few hundred lines, this portion, having little defining 

 power, takes no part in the formation of the spectra actually 

 seen, but simply scatters light, and of course introduces an error 

 in the average of grating-space obtained by measuring the whole 

 ruled surface. 



The gratings used by the writer were therefore calibrated, and 

 corrections calculated and applied to the above wave-lengths, 

 reducing them to 5896-04 and 5896 "09. The mean value taken 

 was 5896 '08. 



On subjecting Pierce's gratings to a like examination, values 

 nearly coincident with the above were obtained. During the 

 present summer an admirable thesis by Dr. Kurlbaum has 

 appeared giving results quite close to the writer's — about 

 5^95*93 for the mean of two gratings, uncorrected, however, 

 for errors of ruling. 



We can, from the close agreement of the results obtained 

 by Kurlbaum, Pierce (corrected), and the writer, feel sure that 

 the wave-length of D is very near to 5896 "00, and consequently 

 all wave-lengths based on Angstrom's value are incorrect by at j 

 least one part in 8000. But this would not be a very serious 

 matter if Angstrom's relative wave-lengths were exact, which 

 they are not. 



On the Existence of Reflection when the Relative Refractive 

 Index is Unity, by Lord Rayleigh, Sec. R.S. — He wished to 

 find whether there was any reflection from a plate of glass 

 immersed in a liquid of the same refractive index as the glass. 

 The liquid used was a mixture of carbon bisulphide and benzole, 

 and it was contained in a hollow prism. The glass plate was 

 roughened behind to get rid of the second surface of reflection, 

 and was mounted in the prism. 



It was found that when the index was the same there was 

 nothing like abolition of the reflection. The flame of a candle 

 could be seen distinctly reflected in the glas*--. The phenomenon 

 may be better followed by mounting the glass in such a way that 

 it is possible to pass from a grazing to a perpendicular incidence. 

 The ray for which the refractive index is made the same being 

 chosen about the middle of the spectrum, as you alter the 

 obliquity of the light, total reflection occurs for either end of 

 the spectrum, and a dark band occupies the middle region. No 

 doubt this band appears dark by contrast. In this way one 

 could be certain that the index had really been equalized. 



He next tried freshly- polished glass, and the reflection from 

 it was not more than one-fourth of that from old gla-^s, although 

 the latter had been carefully cleaned. Still even from the 

 polished glass the reflection was very copious. It did not need 

 any care of adjustment in order to get the reflection of the candle- 

 flame. The light so reflected was not coloured. There was a 

 moderate reflection of all kinds. He confirmed this by using 

 sunlight. 



Where dispersion exists there is no reason to suppose that 

 reflection should cease merely because the refractive index is 

 equalized. If recently-fractured glass should give the same 

 result, one might safely conclude that there was no residual 

 film in play, and there would then be no doubt of the inaccuracy of 

 Fresnel's law. 



On the Action of an Electric Current in hastening the Forma- 

 tion of Lagging Coinfoicnds, by Dr. Gladstone, F.R. S. — The 

 influence of the current was tried on various solutions from 

 which, under normal conditions, precipitation takes place only 

 slowly. A mixture of tartaric acid ^nd potassium nitrate, a 

 mixture of potassium oxalate and magnesium sulphate, a mixture 

 of calcium sulphate and strontium nitrate, and some other mix- 

 tures were used. It was demonstrated that the current does 

 hasten the action. 



NOTES. 



The International Hygienic Congress at Vienna (attended by 

 no fewer than 2250 members) was opened on Monday by the 

 Austrian Crown Prince, who in a brief address referred to the vast 

 importance of hygiene. After the Crown Prince's speech, which 

 was much applauded, Prof. Griiber, the Hon. Secretary, read a 

 report on the organization of the Congress. Two addresses 

 were delivered by M. Brouardel and Herr Pettenkofer, the 

 formerof whom spoke on typhus abdominalis,ihQ latter on hygienic 

 instruction in Universities and technical schools. M. Brouardel 

 said that the disease of which he spoke is far more dangerous 

 to man than cholera, and that it is still an open question whether 

 it owes its origin to the decomposition of organic matter or 

 whether there is a specific virus. He maintained that in 80 

 cases out of 100 typhoid fever is caused by polluted water, and 

 that the question of water supply must always take a foremost , 

 rank in hygienic administration. Herr Pettenkofer lectured 

 hygienic instruction in Universities and technical schools, ancj 

 dwelt on the necessity of spreading hygienic principles among 

 all classes of society. He largely quoted English authoritiesj 

 and, in alluding to the English proverb "Cleanliness is next tc 

 godliness," remarked that the statistics of the mortality 

 London .show how hygienic piety has been rewarded by the 

 heavens. In the course of his address he dealt with the question 

 of quarantine arrangements. He denied that the English ard 

 responsible for cholera coming to Europe through the Suez CanalJ 

 " This opinion," he said, " is clearly refuted by the fact that w« 

 were frequently visited by the disease before the Suez Canal wa^ 

 opened, and that since that time the epidemic has appeared ir 

 many European countries, while Great Britain, which now stand^ 

 accused, and has suffered much through cholera in former timesJ 



