April 1, 1890.] 



KNOWLEDGE 



117 



numerous varieties of oranges, lemons, and citrons grow- 

 ing in India, with a view of encouraging the development 

 of what he believes might be made a very important 

 industry, in a country wlaere labour is cheap, as com- 

 pared with California and the Mediterranean littoral, and 

 the climatic conditions are in no way inferior. It seems 

 that many of the Indian varieties grow with little or no 

 artificial irrigation. Tliey are mostly propagated from 

 seed, sown thickly in a seed nursery on prepared earth, 

 till the plants are about four inches high, when they are 

 transplanted and placed eight or nine inches apart, and 

 allowed to grow till they are four years old ; they are then 

 again transplanted to the orange gardens where they 

 are intended to grow. The bool\, though intended for 

 specialists, contains a great deal of interesting informa- 

 tion for others besides professional orange-growers. It is 

 accompanied by an Atlas of over 250 plates, showing a 

 remarkable variety in the forms and sizes of fruit ; and 

 Dr. Bonavia has given an excellent Index. 



Our Ei/es, and How to Preserrc them from Tntiincij to Old 

 Aqc. By .loHN Browning, F.R.A.S. Eighth Edition. 

 (Chatto & Windus.) This is a very useful little book, 

 explaining in simple language, and by means of \ery 

 numerous illustrations, facts which ought to be widely 

 known in this age of over-taxed eyes. Besides the 

 ordinary explanations as to short sight and astigmatism, 

 and other physical defects of our eyes, it contains some 

 sensible suggestions as to the proper fitting of spectacles, 

 and minor matters which are too little attended to. 



The Educational Annual. 1890. Compiled by Edwaed 

 Johnson. (George Philip & Son.) This is a useful com- 

 pilation, putting into small compass a large amount of 

 information on all kinds of echicational matters pertauiing 

 to all grades from the elementary schools to the universi- 

 ties. The chief deficiency we notice is in connection with 

 secondary education. Wliile good lists, with abundant 

 details, are given of the proprietary and endowed schools, 

 the private schools are almost entirely ignored. And yet 

 many of these, whatever may have been their condition in 

 past years, are now doing excellent educational work, as 

 is testified by the extent to which they figure ha the Uni- 

 versity Local Examinations. 



We regret to announce the death of Prof. Montigny, 

 well Itnown for his researches on the scintillation of stars, 

 on the IGth of March. He died at Schaerbeek, near 

 Jirusscls, after a short illness. He was born in January 

 1S1!), and was consequently in his 72nd year. 



Till' photograph of a sun-spot given in the Fcbniaiy 

 nuiiilier of KxowLEDCK was enlarged from a photograph 

 taken by M. .lansseii on the 2Sth of Juno 1885. On the 

 scale of the enlargement the sun's disc would be 1-40 

 meters in diameter. 



llrttcvs. 



C'l'lio Kilitor iloos not hold himself rosponsiblo foi- the opinions or 

 statements of eoiTcsiionilonts. ] 



ASTU(iN()MlCAI. I'llOTItiiHAlMIY. 

 'I'd the h'.ditor of Knowi.kdge. 



1)i:a1! Silt, - 1 beg you to accept my thanks for a copy of 

 Kno\vi,ki)i;k containing yoiu- interesting article on stellar 

 parallax, and Mr. Sadler's valuable collection of results, 

 the most complete that 1 have seen. 



Your cx-iticism of the accuracy of results obtained from 

 photographs is timely, since this method needs as careful 

 examination as any other. There is a tendency at present 

 to extol the exactness of the photographs without stopping 

 to measure and discuss them, and we already have a de- 

 mand for fire-proof buildings in which to stow the plates. 

 Thus astronomy is in danger of being reduced to the con- 

 dition of meteorology, where a million observations more 

 or less are of no account. In 1874 I was shown a ishoto- 

 graph of Venus /)( tran.situ, with the remark, One photoiiraph 

 is sujticient '. as though a few photographs would settle 

 for ever the distance of the sun The discussion of the 

 American photographs of the transits of 1871 and 18K2, 

 nearly two thousand in' number, gives for the probable 

 error of the position of Venus from a single plate about 

 half a second of arc ; that is, the photographic plate has 

 approximately the same degree of accuracy as an observa- 

 tion with a meridian circle. Our photographs of these 

 transits are beginnmg to deteriorate, and it is well that the 

 plates have been measured and the results secured. 



Still, it' appears to me that the photographic method 

 does furnish certain real advantages for the determmation 

 of stellar parallax. These come largely, I think, from the 

 fact that we have on the same plate stars of comparison in 

 various directions and distances, and that by means of these 

 we are enabled to eliminate constant errors, which are the 

 most dangerous in this kind of work. The hehometer pre- 

 sents similar advantages, but the photographs have over this 

 instrument the advantage of much greater ease in observ- 

 ing. It remains to be shown what degree of accuracy can 

 be obtained from recent photographs in measuring relative 

 positions. The results published from the Potsdam ob- 

 servatory indicate an accuracy equal to that of the best 

 micrometrical work. — Yours trulv. 



U.S. Naval Observatory, Washington. 

 Feb. 27, 1890. 



As.VPH H-VLL. 



[I fully concur with Prof. Asaph Hall as to the advan- 

 tages oft'ered by the photographic method, if the probable 

 error of a single measure, made on a photograph, can be 

 relied on as comparable with the probable error of a single 

 eye observation, and I do not doubt that the average de^•ia- 

 tion from the mean of series of measures made on photo- 

 graphs have been found to be comparable \\\%\x the a\erage 

 deviation from the mean of eye observations ; but we can 

 only assume that the average deviation from the mean is 

 the probable error of a single observation when all syste- 

 matic errors are eliminated. The most practical test one 

 can apply to determine whether all constant sources of 

 error liave been eliminated is the comparison of measures 

 made by independent obser\ers on different photographs. 

 When several such measures confirm one another witliin 

 the limits of probable error claimed, my faith iu the 

 photographic method will increase. — A. C. llANVAiiD.] 



To the Editor of liNowxEDGE. 



SiK, — May I be allowed, as an old variable star observer, 

 with all deference, to suggest a doubt whether the diagram 

 in your March number, with its marked angularities, pre- 

 sents a true picture of the light curve of N. Ursa; 

 ^[ajoris .' The Kousdon observations seem to me to be too 

 loosely scattered to enable one to distinguisli with pre- 

 cision between accidental errors of observation and real 

 points ou the curve. — 1 am, Sir, yours foithfully, 



Geukge Knott. 



Knowle's Lodge, Cucktield, March 21, 1890. 



[One would naturally listen with deference to any criti- 

 cism fi'om Mr. Knott with respect to the observation of 



