I^ov. 8, 1877] 



NATURE 



23 



soon found that this small aperture, as was to be expected, 

 produced poor images in consequence of the diffraction 

 effects brought about by it. It then became a question 

 of increasing the aperture while the exposure was reduced,, 

 and many forms of instantaneous shutters have been 

 suggested with this end in view. With these, if a spring 

 be used, the narrow slit ' which flashes across the beam 

 to pay the light out into the plate changes its velocity 

 during its passage as the tension of the spring changes. 

 Of this again Dr. Janssen has not been unmindful, and 

 he has invented a contrivance in which the velocity is 

 constant during the whole length of run of the shutter. 



By these various arrangements the plates have now 

 been produced at Meudon of fifteen inches diameter, 

 showing details on the sun's surface of less than one 

 second of arc. 



So much for the modus operandi. Now for the branch 

 of solar work which has been advanced. 



It is more than fifteen years ago since the question 

 of the minute structure of the solar photosphere was one 

 of the questions of the day. The so-called "mottling" 

 had long been observed. The keen-eyed Dawes had 

 pointed out the thatch-like formation of the penumbra of 

 spots, when one day Mr. Nasmyth announced the dis- 

 covery that the whole sun was covered with objects 

 resembling willow leaves, most strangely and effectively 

 interlaced. I here quote from Sir John Herschel.® 



" According to his observations, made with a very fine 

 telescope of his own making, the bright surface of the 

 sun consists of separate, insulated, individual objects or 

 things, all nearly or exactly of one certain definite size 

 and shape, which is more like that of a willow leaf, as 

 he describes them, than anything else. These leaves 

 or scales are not arranged in any order (as those 

 on a butterfly's wing are), but lie crossing one another 

 in all directions, like what are called spills in the 

 game of spilikins ; except at the borders of a spot, 

 where they point for the most part inwards, towards the 

 middle of the spot, presenting much the sort of appear- 

 ance that the small leaves of some water-plants or sea- 

 weeds do at the edge of a deep hole of clear water. The 

 exceedingly definite shape of these objects ; their exact 

 similarity one to another ; and the way in which they lie 

 across and athwart each other (except where they form a 

 sort of bridge across a spot, in which case they seem to 

 affect a common direction, that, namely, of the bridge 

 itself), all these characters seem quite repugnant to the 

 notion of their being of a vaporous, a cloudy, or of a 

 fluid nature. Nothing remains but to consider them as 

 separate and independent sheets, flakes, or scales, having 

 some sort of sohdity. And these flakes, be they what 

 they may, and whatever may be said about the dashing 

 of meteoric stones into the sun's atmosphere, &c., are 

 evidently the immediate sources of the solar light and 

 heat, by whatever mechanism or whatever processes they 

 may be enabled to develop, and as it were elaborate these 

 elements from the bosom of the non-luminous fluid in 

 which they appear to float. Looked at in this point of 

 view, we cannot refuse to regard them as organisms of 

 some peculiar and amazing kind . . . . " 



* Here, then, was a discovery with a vengeance ! and 

 absolute endorsement from the man above all others who 



» I have recently been making some experiments with a view of getting 

 rid of the narrow aperture in general use, as it has appeared to me that the 

 diffraction effects produced by it must be as injurious to definition as those 

 due to a small object-glass. I have found that a circular aperture, allowing 

 the whole beam to be flashed on the plate in conjunction with a plate of 

 optically pure yellow glass nearly ia contact with the photographic plate can 

 he used without over-exposure. 



2 " Familiar Lectures," p, 87, 



had a right to express an opinion. Nevertheless, the 

 organisms have since disappeared, and the work of 

 many careful observers has established that the mottling 

 on the sun's surface is due to dome-like masses, 

 and that the " thatch " of the penumbra is due to these 

 dome-like masses being drawn, either directly or in the 

 manner of a cyclone, towards the centre of the spot. In 

 fact the "pores "in the interval between the domes are 

 so many small spots, while the faculae are the higher 

 levels of the cloudy surface. The fact that faculas 

 are so much better seen near the limb proves that the 

 absorption of the solar atmosphere rapidly changes 

 between the levels reached by the upper faculae and the 

 pores. 



These masses are in all probability due to a rapid 

 increase of pressure in the portion of the solar atmo- 

 sphere occupied by the photosphere; we know, or think we 

 know, that they are not due to reduction of temperature. 



Thus much presumed we now come to Dr. Janssen's 

 discovery. 



An attentive examination of his photographs shows 

 that the surface of the photosphere has not a constitution 

 uniform in all its parts, btit that it is divided hito a series 

 of figures more or less distant fro7n each other, and pre- 

 senting a peculiar constitution. These figures have con- 

 tours more or less rounded, often very rectilinear, and 

 generally resembling polygons. The dimensions of these 

 figures are very variable ; they attain sometimes a minute 

 and more in diameter. 



While in the interval of the figures of which we speak 

 the grains are clear, distinctly terminated, although of 

 very variable size, in the interior the grains are as if half 

 effaced, stretched, strained ; for the most part, indeed, 

 they have disappeared to make way for trains of matter 

 which have replaced the granulation. Everything indi- 

 cates that in these spaces, as in the penumbras of spots, 

 the photospheric matter is submitted to violent move- 

 ments which have confused the granular elements. 



In an article recently contributed by Dr. Hunter and 

 myself to the Nineteenth Century^ the following pas- 

 sage occurs : — 



" The spots may be taken as a rough index of solar 

 energy, just as the rainfall may be taken as a convenient 

 indication of terrestrial climate. They are an index but 

 not a measure of solar activity ; and their absence indi- 

 cates a reduction, not the cessation, of the sun's energy. 

 Whether this reduction means one in a hundred or one in 

 a thousand we do not ktiow." 



With the same idea in his mind Dr. Janssen points out 

 that this fact throws light upon the forms of solar activity, 

 and shows that that activity, in the photosphere, is always 

 very great, although no spot appears on the surface. 



We have already referred to the paradox that the sun's 

 appearance can now be best studied without the eye 

 applied to the telescope. This is what Dr. Janssen says 

 on that point. 



The photospheric network cannot be discovered by 

 optical methods applied directly to the sun. In fact, to 

 ascertain it from the proofs, it is necessary to employ 

 glasses which enable us to embrace a certain extent of the 

 photographic image. Then if the magnifying power is 

 quite suitable, if the proof is quite pure, and especially it 



I " Sun-spots and Famines," Nineteenth Cenfufy, November, \Zn, p. 584- 



