OcTOBKii 2, 1893.] 



KNOWLEDGE 



189 



WHAT IS THE SUN'S PHOTOSPHERE ? 



By A. C. Ranyakii. 



PERHAPS it may be well to explain, for the sake of 

 readers who are not familiar with the long names 

 which have been given to various regions of the 

 sun, that the photosphere is the shining sphere 

 we generally speak of as the sun. It is in fact 

 only a part of the sun, but it is the part with which we 

 are most familiar, because it can be seen with the naked 

 eye, and can be examined with the telescope under ordi- 

 nary conditions. 



Immediately outside the photosphere is a region in which 

 coloured prominences can be detected with the spectroscope 

 under ordinary conditions. This region immediately above 

 the photosphere is called the chromosphere ; it has no 

 definite outer limit, though it has a comparatively sharply 

 defined lower limit, marked out by the upper surface of 

 the photosphere. During a total solar eclipse the chromo- 

 sphere, or the upper portions of the chromosphere 

 which are not hidden by the moon, can be seen with the 

 naked eye and with an ordinary telescope, extending to a 

 greater height above the sun's limb than they can be 

 traced with the spectroscope under ordinary daylight condi- 

 tions; and towering to a vast height above the chromosphere 

 are still fainter structures and rays, extendmg sometimes 

 to a distance of more than a solar diameter from the moon's 

 limb. The faint structures extending into the outer region 

 are collectively spoken of as the corona, but there is no 

 defined limit separating the corona from the chromosphere. 

 Bright Imcs, indicating the presence 'if hot vapour, can be 

 detected with the spectroscope extending to a great height 

 in the corona, and the structures of the corona can be 

 traced down through the region of the photosphere in a 

 manner which would lead us to conclude that the coronal 

 structures, as well as the prominence structures of the 

 chromosphere, have their origin in a region below the 

 bright surface of the sun. 



To define the photosphere as the highly luminous 

 spherical surface which surrounds the nucleus of the solar 

 nebula is not to explain what it is. The beautiful photo- 

 graphs of parts of the solar photosphere which, by the 

 kindness of Dr. Janssen, I am able to lay before readers of 

 Knowledoe, show that the shining surface is broken up 

 into a series of masses arranged with a certain degree of 

 parallelism in adjacent parts, like the cirrus clouds in a 

 mackerel sky ; and in Plate I., which represents the 

 photosphere on a larger scale than Plate II., it will be 

 S3en that the elongated bright masses are striated across 

 their length, and that in many cases they break up into a 

 series of rounded cloud masses arranged in rows. 



I have had two classes of plates printed — one in a 

 chocolate-brown ink, which has been over-exposed in 

 preparing the plate for printing so that it only shows the 

 brightest parts of the photosphere, and another in a dark 

 platinotype tinted ink with less exposure, so that the details 

 of the brightest regions are sacrificed in order to show 

 fainter and more nebulous parts. Those who take an 

 interest in the matter will be repaid by comparing the two 

 Classes of plates. About half of the edition of Knowledge 

 hj,s been issued with the chocolate-brown plates, and the 

 remainder with the dark platinotype tmted plates. 



It was long ago pointed out by Dr. Janssen that a 

 reticulated pattern is traceable in the arrangement of the 

 cloud masses of the photosphere. In the regions corre- 

 sponding to the strings of the netting the cloud masses are 

 clearly defined. They are separated by numerous dark 

 interspaces, and the elongated masses appear to stand up 



or to be arranged more or less radially to the sun's centre ; 

 whereas in the areas corresponding to the meshes of the net 

 the structure appears to be hazy or blurred, and the elon- 

 gated cloud masses lie more or less parallel to one another 

 as if raked out in a plain at right angles to the solar radius. 

 The phenomena presented by solar spots and the dark 

 interspaces between the clouds of the photosphere would 

 lead us to conclude that the photosphere is a comparatively 

 thin layer of brilliantly shining clouds, which is torn and 

 churned by innumerable currents, as indeed wo might 

 expect if all the uprushing streams of the chromosphere 

 and corona pass through it. 



In the very interesting lecture on "Flame," delivered 

 before the recent meeting of the British Association at 

 Nottingham, Prof. Smithells concluded his address by 

 asking his audience to consider what must have been the 

 condition of the earth in past ages. 



"The earth," he said, "is known to be a cooling body, 

 and also an oxidized body. At one time it must have been 

 too hot for the cceans to have existed upon it in a liquid 

 state, and at a still more remote period all the waters of the 

 earth probably existed as an enormous gaseous envelope of 

 uncombined hydrogen and oxygen. Chemistry forces us 

 to imagine an intervening time at which this oxygen and 

 hydrogen would begin to combine. During that period, 

 huge cosmical flames would rend the atmosphere. The 

 steam formed would descend to the hotter strata of the 

 pre-geologic atmosphere, would be dissociated and sent 

 forth again to combine in the upper atmosphere, causing 

 an incessant celestial pyrotecliny," which the lecturer 

 concluded must have caused the earth — as seen from a 

 distance — to resemble the sun as known to solar physicists 

 at the present time. 



I am not able to adopt Prof. Smithells' theory as to the 

 constitution of the solar photosphere, but it seems to me 

 that too little attention has been devoted to the explosive 

 combinations which Prof. Smithells points out must take 

 place in some region of a highly-heated atmosphere of 

 mixed gases. Since the day when Sir John Herschel 

 calculated the heat which would be given out during the 

 burning of a globe of coal as large as the sun, and compared 

 it with the heat which is given out by the sun in six 

 thousand years, it has been too frequently assumed in 

 popular astronomy books that no chemical combinations 

 can be taking place upon the sun ; but without chemical 

 combination and explosions it is very difficult to conceive 

 of an adequate cause for the swift uprushes of matter we 

 observe in the chromosphere, and the still more gigantic 

 uprushes whose existence is evidenced by the structures 

 observed in the corona. 



The great deviation from the radial of many of the 

 streams of matter projected into the chromosphere, as 

 well as the tangential rays of the corona, point to the 

 conclusion that the region of these explosions cannot be 

 situated at any very great distance below the photosphere. 

 It is possible that such explosions may take place in the 

 region of the photosphere, but it does not seem probable 

 that the intense light of the photosphere can be due to 

 burning or chemical combination, for we know of no flames 

 comparable in brightness with the brilliant incandescence 

 of the electric arc light, and the mcandescent carbon 

 candle of an electric light is not as bright as the solar 

 photosphere. The most brilliant lights we know of are 

 produced by the intense incandescence of solid substances. 

 Thus a mass of lime in the dull oxy-hydrogen flame is 

 brighter than any gas flame we can produce, but, according 

 to the measures of Foucault and Fizeau in 1844, the solar 

 surface was found to be one hundred and furty-six times 

 more brilliant than the calcium light ; and Prof. Langley 



