Mar., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



47 



operation to some of the above places, will give some 

 indication of the time one should allow. 



Burgos and Palenci 

 London, 2.20 p m. 

 Paris, 10.7 p.m. 

 Paris, 10.36 p.m. 

 Burgos, 9.33 p.m. 

 V'enta de 



Banos, 11. 15 p.m. 

 Venta de 



Banos, 1.46 a.m. 

 Palencia, 2.1 a.m. 

 Santander, — 

 London to Barcelona 

 {via Lyons^ 



h. m. 

 3t 46 



29 J hrs. 



Atf.ca ani> Calatavud 



London, 90 p.m. 



5 50 a.m. 1 



12 iS p.m 



2.25 p.m. 



2.50 p.m. 



Paris 

 Paris, 

 Madrid 

 Madrid 



h. m. 



5G 50 



Ateca, 



4.20 a.m. 



Calatayud, 4.50 a.m. 

 Saragossa, — 



Barcelona to Castellon 

 (viii Tarragona) 



6J hrs. 



The work to be attempted at any of the stations named 

 has to be done in about 3^^ minutes, hence to obtain 

 useful results one should have a knowledge of some of 

 the problems requiring further investigation or of new 

 points to be attacked and arrange a programme 

 accordingly. 



The aim of all eclipse expeditions is to study those 

 parts of the sun which are visible only during a total 

 eclipse, in order to .gain a further insight into the physical 

 condition of the sun as a whole, and ultimately to bring 

 this knowledge to bear on other cosmical bodies in 

 general. The most obvious feature of a total eclipse is, 

 of course, the Corona, and although it has been so re- 

 peatedly assailed, deeper and more perplexing problems 

 have arisen in proportion to the assaults, and Professor 

 Campbell concludes that " it is as enigmatical as ever." 



Some of the more interesting points regarding the 

 Corona are : — 



Its visibility, photographically or visually, in the partial 

 phases of the eclipse. 



The extension of the coronal rays. 



The differentiation of the part which shines mostly by 

 reflected light from that which is self-luminous. 



Its spectrum. 



Other points, such as its connection with prominences, 

 dark rifts, detailed structure, photometric value at vary- 

 ing distances from the Moon, rotation, brightness and 

 wave lengths of its spectrum lines, cannot be fully con- 

 sidered in an article of this length. 



Small cameras can with ad\'antagebe employed in the 

 solution of the first three points, but to meet with any 

 measure of success the lens ratio of aperture to focal 

 length requires to be large, partly in order that the ex- 

 posure may be short enough to neglect equatorial follow- 

 ing which will be unprovided for in the majority of cases 

 with this class of instrument. 



The most notable result as to coronal extension is the 

 photograph obtained by Mrs. Maunder at the eclipse of 

 1898, with a lens ratio of /'6 exposing 20 seconds on a 

 triple-coated Sandell plate, but as the conditions of the 

 coming eclipse are different and the sun is approaching 

 a maximum, long rays may not exist, or if present be no 

 brighter than the sky background. However, it is in this 

 direction that one must look for the recording of rays to 

 the extent that the naked eye is able to perceive them. 

 For prominences and the recording of detail of the 

 lower parts of the Corona, a lens of long focal length, 

 slow plates, and exposures of half a second or less are 

 preferable, but as the scale of ordinary cameras is small, 

 such records are best obtained with larger instruments, 

 as the every-day camera is more profitably employed on 

 other work. 



Photographs should be taken several ininutes before 

 and after totality to ascertain when the Moon's limb can 

 be discerned beyond the arc shown on the solar disc ; 

 when this point arrives it is evident we are seeing the 

 Moon projected on the Corona as a background. It 

 would be desirable to prevent the direct image of the 



crescent sun falling on the plate, but as this is varying, it 

 is scarcely possible, except with a large image and clock 

 movement, though one might try the effect of a screen 

 such as is used in cloud photography, one of a greenish 

 hue being preferable. 



Such photographs will furnish useful data to determine 

 the feasibility of observations of the Corona in annular 

 eclipses or even without an eclipse. 



In consequence of the varying brightness of the Corona 

 at different distances from the sun's centre, it has usually 

 been found necessary to compile a composite picture 

 from many negatives to exhibit the detail of the coronal 

 extensions from the sun's limb to their extremities, since 

 any one exposure will only be correct for a particular 

 brightness, some parts being over and others under- 

 exposed. Now, the type of Corona to be expected is 

 that usually shown at the maximum sun-spot period, and 

 will in all probability be similar to those of 1882 and 



1893; Professor Turner has shown that in the case of 

 the latter the law of luminosity of the Corona was : 

 . , varies / distance from \~(> 



Brightness ^^ (^ sun's m,t re ) 



He has shown also that the same law represents the 

 luminosity of the 1898 Corona, and probably it ap- 

 proaches near the truth for all the various types of 

 Coronas, and is more satisfactory than that formulated by 

 Professor Harkness in 1878, who gave the brightnesf 

 varying simply as the inverse square from the hmh, for 

 if the Corona be largely made up of minute particles, 

 other inverse powers of the distance in addition to the 

 ordinary inverse square law of luminosity will enter into 

 the equation. 



Hence to obtam photographs of the Corona with the 

 exposure correct for every part, it will be necessary to 

 adopt a method similar to that employed by Buckhalter, 

 and use a rotatmg disc immediately in front of the plate 

 on which the image falls, with a templet cut out to give 

 the exposure in accordance with the above law. 

 (To he Continued.) 



Exhibition of Meteorological Instr\iments. 



The Council of the Royal Meteorological Society have 

 arranged to hold, by permission of the President and Council 

 of the Institution of Civil Engineers, at their house in Great 

 George Street, Westminster, an exhibition of meteorological 

 instruments from March 14 to 17 next. The exhibition vvill 

 be chiefly devoted to recording instruments; but will also 

 include new meteorological apparatus invented or first con- 

 structed since the Societv's last exhibition, as well as photo- 

 graphs, drawings, and other objects possessing meteorological 

 interest, or instruments of very early origin. 



