• KNOWLEDGE 



[Jan. 



18b3 



of judgment than ho did on the particular point I have 

 Ix'pn considering. 



How far this error of judgment affected the result is a 

 matter about which I offer no opinion — though of course 

 I have one. 



PHOTOGRAPHING THE SOLAR 

 CORONA.* 



ASTllONOMERS ha\ c long hoped, almost against hope, 

 for a time to come when the solar corona might be 

 seen, even as since 18G8 the sun's coloured prominences 

 have been soon, without the aid of a total solar eclipse. 

 Almost against hope, because what is known about the 

 corona shows that its light is far too faint to be discei-ned 

 directly, so greatly is it overmastered by the light of the 

 .sky near the sun, while the method which avails to show 

 the coloured prominences cannot be successfully applied 

 to the coi'ona. We know that, during a solar eclipse, 

 even the brighter part of the corona is only seen a few 

 seconds before totality begins, and can only be discerned 

 for a few seconds after the total phase is past. In the 

 full glory of sunlight it is as hopeless to look for the 

 corona, either with or without a telescope, as it 

 would be to look for a star of the fourth or fifth 

 magnitude without telescopic aid in the daytime. 

 And what a telescope does to make a star visible in the 

 daytime cannot be done for the corona ; the star is but a 

 point even when the most powerful telescope is turned on 

 it, and therefore a star looks so much the brighter, not so 

 much the larger, as the illuminating power of the telescope 

 is increased. But an object like the corona, or a comet, or 

 a nebula is magnified in at least as great a degree as the 

 illuminating power is increased, and in yet greater degree 

 ■when high magnifying powers are used, so that it looks no 

 brighter, only larger; in fact, as some light is always lost 

 in passing through the lenses, the apparent brightness of 

 such objects is alwajs rather reduced when a telescope is 

 used, however greatly the total quantity of light received 

 from them may be increased. 



The same is true of the sun's coloured flames, so that it 

 was long regarded as hopeless to look for them at any other 

 time but during the few minutes' duration of total solar 

 eclipse. But spectroscopic analysis, so soon as it was shown 

 that they shine onl}' with special tints, gave tlie means of 

 seeing them in broad daylight. If we consider how this 

 was done, we shall better understand the difliculty in 

 regard to the corona. We cannot do this better, though 

 that is not the actual manner in which the observation is 

 made, than by considering Newton's familiar experiment 

 on the decomposition of light. In that, light admitted 

 through a circular opening, being dispersed by a prism, 

 cast a long rainbow-tinted image on a screen, this image 

 being really made up of multitudinous images of all the 

 colours, not of the rainbow, which is usually but a diluted 

 spectrum, but of the true solar spectrum. In that long 

 streak there were thousands of images of various tints of 

 red, thousands of orange images, thousands of yellow, 

 green, blue, indigo, and violet images, so many, in fact, 

 that no eye could see where any single image began or 

 ended : all were completely bli^nded together. The light, 

 which, undispersed, would have formed a single circular 

 white image, being spread over a long ril)bon-like]band of 

 space, was, of course, correspondingly weakened ; no part 

 of the spectrum was nearly as bright as the round white 

 spot seen before the prism was interposed. Now, sup- 



*From the Times'. 



pose that instead of shining with all the colours of the 

 rainbow, the sun shone only with a certain small number of 

 tints — say, on(^ kind of red oidy, one kind of orange, and 

 one kind of blue. Then, instead of the rainbow-coloured 

 streak formed by tens of thousands of blended images, 

 there would have been but three images — three circular 

 discs — a red circle, an orange circle, and a blue circle. 

 The light, which before dispersion had formed a single 

 image, would liave been weakened in forming three, but 

 not nearly so much as in tlie actual case, where the long 

 streak formed by multitudinous images was many times 

 longer than the diameter of a single circular image. 

 And any further dispersion, which in the actual case 

 would have correspondingly lengthened the spectrum 

 and weakened its lustre, would only have thrown the 

 three images further apart, leaving their brightness un- 

 changed. This diOerence actually exists between the 

 white light of the sunlit sky and the light of the sun's 

 coloured prominences. These are masses of glowing gas 

 shining chiefly with one red tint, one orange-yellow tint, 

 and one green-blue tint. If we receive on a screen (or 

 when we receive on the retina as on a screen) the light 

 from the sky over a prominence and the light from that 

 great mass of glowing gas, we are unable to distinguish the 

 shape of the prominence, because its light is quite over- 

 mastered by the light of the sky ; but when, by means of 

 suitably-arranged prisms, we disperse the light thus re- 

 ceived, we weaken the sky-light just in proportion as we 

 increase the dispersion ; while we only throw the images of 

 the prominence further apart with each increase of spec- 

 troscopic dispersion ; so that if only our spectroscope has 

 sufficient dispersive power, the prominence, no longer forced 

 to contend against the whole light of the sky, but only 

 against a small portion of it, becomes discernible on the 

 weakened light of the rainbow-tinted background. 



If the corona were of the same nature as the pro- 

 minences, its form might be rendered discernible in the 

 same way. But, as a matter of fact, only the very brightest 

 part of the corona shines with light of special tints ; the 

 greater part of this stupendous solar appendage shines with 

 light of all the colours of the rainbow, though, fortunately, 

 with an excess of light from certain parts of the spectrum, 

 otherwise the success we have now to record would never 

 have been achieved. 



We must here make a few remarks respecting ordinary 

 methods of observation, and theii failure to show any trace 

 of the corona. It occurred long since to 8ir George Airy 

 to receive the image of the sun and surrounding parts of 

 the sky on a smooth white surface, and by removing that 

 part of the surface on which the image of the sun 

 itself fell, and putting thei-e a black surface (as it were 

 "quenching the sun in a black bag") to give the pro- 

 minences and corona a better chance of being seen. 

 The method failed utterly. It was later suggested 

 that so far as the prominences were concerned, the 

 use of a ruby-tinted medium cutting off all the 

 light except rays of nearly the red tint of the pro- 

 minences, and the substitution of a smooth red 

 surface for a white one, would greatly help to make them 

 visible. It was further suggested that if, when both devices 

 had been employed, the light received on the coloured card 

 were examined through a spectroscope of suitable power, 

 the whole ring of coloured prominences around the sun 

 might be seen at a single view. And it is worthy of notice 

 that the first coloured prominence ever seen — as a whole — 

 by man, without the aid of an eclipse, was seen by Mr. 

 William Huggins, by the aid of the absorptive power of 

 ruby-tinted glass. But, so far as the corona was concerned, 

 such methods as these had persistently failed. 



