October i, 1891] 



NATURE 



527 



of the Potsdam Observatory is a sufficient guarantee, 

 Dr. Scheiner presents the following table, in which is 

 exhibited the faintest magnitude which, under certain 

 varied circumstances, can be detected on a photographic 

 plate : — 



It will be noticed that while each successive exposure 

 is 2'5 that of the preceding, the corresponding gain in 

 light is considerably less than one magnitude. From 

 each of the four plates the gain is as follows : — 



Plate 



IT. 

 III. 

 IV. 



Gain in mag. 

 053 



o-6i 

 o'5o 

 0-48 



The mean is 0-53 — that is to say, instead of one 

 magnitude being gained by continued exposure through 

 each successive interval, the actual gain is only half a 

 magnitude. The exception that might be taken to these 

 experiments is, that the detection of the feeblest stars on 

 a plate is a matter of doubt and great practical difficulty. 

 Dr. Scheiner has, however, availed himself of a second 

 test by counting the stars on a plate after various expo- 

 sures. With this view two plates were taken of the 

 region round e Orionis, one with an exposure of one hour, 

 the other with eight hours' exposure. Therefore, if 2*5 

 times the exposure produced stars a magnitude fainter, 

 there ought to be a gain of more than two magnitudes on 

 the second plate, and it may be assumed that the number 

 of stars impressed would follow the known law. On the 

 one-hour plate were found 11 74 stars, on the eight-hour 

 5689. There ought to have been on the long-exposed 

 plate over 10,000 stars, so that roughly speaking only one- 

 half of the stars given by the law were photographed. 

 Further, Argelander has catalogued within this area 125 

 stars, and therefore it might have been anticipated from 

 the law of increase that some 10,000 stars would have 

 been visible on the one-hour plate. 



This margin is too great to be readily explained away. 

 Of course, there is the same difficulty in perceiving the 

 minute dots that represent the faintest stars as in the 

 former case, and further, it is possible that the law of 

 average increase of the number of stars did not hold in 

 this particular part of the sky. It is not to be expected 

 that a law, which applies with more or less accuracy on 

 the average to the whole of the sky, is necessarily ful- 

 filled on any small portion, such as the ten-thousandth 

 part. If the stars are not in the heavens, they cannot 

 be photographed. Evidently, it would be unlikely that 

 on every thousandth part of that plate would be found the 

 thousandth part of the total number of stars impressed. 



But allowing for errors of exaggeration and observa- 

 tion, the result is very interesting, and not a little alarming 

 as implying that photography is not so powerful an engine 

 as was at first anticipated, and that, to accomplish the 

 full hope of all that was expected of it, longer exposure 

 and consequently a greater expenditure of time will be 

 needed. Dr. Scheiner gives a little table, which shows 

 that if a star of the 95 mag. be registered in 24 seconds, 

 then in 190 minutes a star of the i6'5 mag. will be photo- 

 graphed, supposing a whole magnitude to be gained by 

 successively multiplying the exposure by 2-5. But if the 

 gain be only 05 in this interval, then the faintest star 

 impressed will be only i3'o mag., even after this long 

 exposure. If 06 of a mag. be the rate of increase, then 

 the I3'6 mag. will be seen ; if 07, then 14-4 mag. The 

 truth will probably be found near this latter limit. 



NO. I 144, VOL. 44] 



NOTES. 

 The second International Folk Lore Congress meets at the 

 rooms of the Society of Antiquaries this afternoon, when an 

 address will be delivered by Mr. Andrew Lang, the President. 

 Three subjects are to be considered— folk tales, mythology, and 

 institutions and customs. To each of these subjects a day will 

 be devoted. The proceedings will be brought to an end on 

 Wednesday morning next. 



The Iron and Steel Institute will meet at the Woolwich 

 Arsenal on Tuesday next. The members are to be conducted 

 over the manufacturing departments at the Arsenal, and will see 

 quick-firing and machine guns in practice. On the following 

 day the Institute will conclude its meeting at the Institution of 

 Civil Engineers. 



The third biennial session of the International Statistical 

 Congress was opened at Vienna, on Monday, by Baron Gautsch, 

 the Austrian Minister of Public Instruction. An address was 

 delivered by Sir Rawson Rawson, the President. 



The seventeenth Annual Congress of the Sanitary Association 

 of Scotland was held in Edinburgh last week. Dr. Farquhar- 

 son, M.P., President of the Congress, delivered an address 

 " On a Model Hygienic State, or a Glance at the Sanitation of the 

 Future." In the course of his remaiks he urged the necessity for 

 more organized attention being given in Parliament to hygienic 

 matters, and advocated the appointment of a Minister of Public 

 Health. 



The Harveian Oration will be delivered at the Royal College 

 of Physicians, by Dr. W. H. Dickinson, at the Royal College of 

 Physicians, on Monday, October 19, at 4 o'clock. 



We referred last week to the death of Prof. W. Ferrel. He 

 was born on January 29, 1817, and since the foundation of the 

 American Meteorological yournal he was a frequent contributor 

 to that paper, from which we take most of the following details 

 of his life. During his boyhood he was kept rather closely at 

 work on his father's farm, and with the first money he earned, 

 he bought a copy of Park's "Arithmetic." Having also a liking 

 for astronomical studies, he used to draw a number of diagrams 

 upon the doors of his father's farm, describing circles with the 

 prongs of a pitchfork. In 1839, he entered one of the Colleges 

 in Pennsylvania, and graduated at Bethany College in 1844. In 

 1857, he became an assistant in the office of the "American 

 Ephemeris and Nautical Almanac," and subsequently entered 

 the U.S. Coast Survey and the Signal Office, from which last he 

 retired in 1886. He was elected a member of the National 

 Academy of Sciences in 1868. Ferrel is described as an ex- 

 tremely diffident man, and he never once sought position ; 

 every official position that he occupied having been offered to 

 him. His first paper bearing directly on meteorology was pub- 

 lished in 1856, with reference to the deflective effects of the 

 earth's rotation upon the motions of the atmosphere ; and this 

 paper, which has done much towards establishing meteorology 

 on a scientific basis, was subsequently revised and reprinted as 

 one of the professional papers of the Signal Service, under the 

 title " Motions of Fluids and Solids on the Earth's Surface." In 

 this treatise he proposed a complete analytical investigation 

 of the general motions of the fluids surrounding the earth. 

 These papers received considerable attention and discussion 

 soon after publication, especially in France ; in America and 

 England they were overlooked until recent years, but they are 

 now recognized as fundamental propositions in the study of 

 meteorology. He also wrote various articles on the tides, which 

 are of equal significance with those on the motions of the atmo- 

 sphere, and he constructed a "maxima and minima tide- 

 predicting machine," which is now in use at the Coast Survey 

 Office in Washington. The last of his numerous works upon 

 meteorology was a " Popular Treatise on the Winds," published 



