72 



KNOWLEDGE >S: SCIENTIFIC NEWS. 



[April, 1905. 



cycle, that the more complex and elaborate structure of 

 the corona at the sunspot maximum may not giveto tele- 

 scopic examinations greater minuteness of detail than 

 any but exceptional photographs can supply." 



In recent eclipses the best photographs taken with the 

 prismatic camera show that the images of the corona in 

 1474 K light are not smooth rings, but rings having a 

 definite form not necessarily coinciding with the outline 

 of the corona as photographed in ordinary cameras, but 

 probably recording a true iiDier corona composed of the 

 unknown gas cnronitim. The disentanglement of this 

 " inner corona," or that part giving foro«/«w/ emanations 

 and self-luminous, from the outer corona, which is com- 

 posed of particles or droplets, and luminous chiefly by its 

 reflective power, is a difficult problem, since even in an 

 eclipse it is the enveloping atmosphere and not a section 

 of it which is presented to us. Here it seems to indicate 

 that advantage must be taken of the outer corona giving 

 a continuous spectrum, whilst the inner corona gives a 

 line spectrum with its principal radiation in the green, 

 the "corona line." If, then, a light filter be employed 



rommence • 



Coronal detail round p 



nee. (Eclipse, 1896.) 



which only allows this green light to pass through in 

 effective quantities, one may succeed in photographing 

 the inner corona alone, and thus determine the distribu- 

 tion of coronium in the corona. Gelatine films stained 

 with aniline blue and tartrazine form a light filter of this 

 nature ; but even this may prove insufficient by itself, 

 and possibly the spectroscope will have to be called in as 

 an additional aid. Suppose, then, a prism from an ordi- 

 nary spectroscope be fixed in front of the lens of the 

 camera, thus making a small prismatic camera which can 

 be accurately focussed by allowing Polaris or a bright 

 star like \'ega to trail and impress its spectrum on the 

 plate; when this is pointed to the eclipsed sun a series of 

 rings partially superposed will result from the radiations 

 of the inner corona, whilst the light of the outer corona 

 will be spread out as a continuous spectrum, and conse- 

 quently enfeebled at any one point ; hence, by the inter- 

 ception of a light filter as above described, one may pos- 

 sibly prevent all but the bright green ring from leaving 

 any record. Another suggestive method, which will 

 enable one to discriminate between these two kinds of 

 radiations, is to take advantage of the fact that light 

 reflected from particles is polarised, and thus, by 

 attaching a Nicol prism in front of the lens, and 

 making several exposures on the corona with the Nicol 

 at various known degrees of rotation, one may be able 

 to sift out the two kinds of radiations. A polariscope 

 opera glass, with the Nicol between the eye and the eye 

 lens, somewhat similar to the prismatic opera glass, would 

 be of service as a supplementary aid. The distribution 

 o{ coronium is a long-standing problem, and, quoting from 

 Professor Eastman's report of the total eclipse of 1878, 

 he says : " The limits of all the known coronal elements 



should be carefully determined by measurement, at each 

 eclipse, and then the study of one important branch of 

 solar physics will rest on definite data. The existence of 

 the ' green line ' has been established for several years, 

 and it is a waste of valuable opportunity to stop at simply 

 saying it was seen." The spectrum of the corona requires 

 in general to be studied with a more powerful equipment 

 than that so far considered. Some observers have 

 reported no Fraunhofer lines in the spectrum of the outer 

 corona, but it would have been more surprising if they 

 had been seen when we learn what apparatus was 

 employed. It is important, therefore, to devote certain 

 apparatus to particular work ; negative evidence is valu- 

 able if obtained with suitable instruments, otherwise it 

 may be misleading. Ade(]uate instruments does not 

 necessarily mean " big," for, referring to the case above, 

 an observer with a powerful spectroscope would in all 

 probability record the absence of Fraunhofer lines, whilst 

 another with a less powerful piece of apparatus would 

 register their presence, and rightly so. 



Yet another instance is the observing of certain 

 phenomenon in one instrument, whilst other means fail 

 to record it. In several eclipses observers using a slit 

 spectroscope have recorded the presence of bright hydro- 

 gen, magnesium, and iron lines in the spectrum of the 

 corona, and yet the prismatic camera failed to show any 

 of these lines. In the slit spectroscope any light falling 

 on the slit will be observable as images of the slit, and in 

 addition to the direct light from the corona and chromo- 

 sphere, there is a certain amount of light derived from 

 the same source, but diffused by particles in our atmo- 

 sphere, which is capable of illuminating the slit sufficiently 

 to be observable. Thus, the bright lines of hydrogen have 

 been observed to extend over the dark disc of the moon. 



The prismatic camera, however, fails to register such 

 spurious radiations of hydrogen and calcium in the 

 corona, as no images of the scattered reflections can be 

 formed. 



The recording of the flash spectrum will form an im- 

 portant item in the programme, both with slit and slitless 

 spectroscopes. It is extremely doubtful, however, 

 whether the spectroscope suitable for this work is also 

 efficient for capturing the spectrum of the corona ; 

 in fact, the " flash " requires both a large image and great 

 dispersion for its disintegration and comparison with the 

 Fraunhofer spectrum, whilst the corona would be more 

 favourably attacked with a small image and high disper- 

 sion, in order that the lines or rings may be intense, 

 whilst the continuous spectrum is either resolved or 

 so enfeebled that the lines are exhibited in better 

 contrast. 



The "shadow bands" are a subsidiary phenomenon, 

 and on this account they have usually been neglected or 

 treated in an unscientific manner; thus we read that their 

 progress was at the rate " of a trotting horse," and 

 observations have usually been confined to one plane. 

 It seems important to gain more definite information 

 about them, whether they are due to scintillatory effects 

 of the atmosphere, in which case wind direction may be 

 important ; or, what is more doubtful, are they some 

 difl'raction effect ? Whatever be the true explanation, 

 it seems necessary to observe them on two planes at 

 right angles to one another, whence the plane in which 

 they lie, or its normal, may be ascertained, and its 

 reference to the sun or the cusps may be determined. 

 Observation should also be made to see if they persist 

 during totality, and especially should they be looked 

 for at any stations of high altitude to note the effect of 

 diminished atmosphere. 



If their movement be too great for accurate eye esti- 



