296 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



and no question any longer remains respecting 

 their reality as solar phenomena. But Myer saw 

 them extending to a distance equal to two or 

 three times the diameter of the lunar disk — say 

 two and a half diameters — which would corre- 

 spond to a distance of more than 2,000,000 miles 

 from the sun's surface. This would greatly ex- 

 ceed anything seen in the photographs, as might 

 indeed be expected. It also considerably exceeds 

 the distance to which other observers have traced 

 the coronal rays. Yet we cannot for this reason 

 reject General Mycr's account, fcr his was not a 

 casual, careless observation, but a careful survey 

 of phenomena by a skillful student of science. 



Instead of calling his observation in ques- 

 tion, therefore, it will be well to inquire under 

 what circumstances it was made ; as it may thus 

 be possible to learn a way by which still more 

 successful observations of the corona may be 

 effected. We find, as we might have expected, 

 that the circumstances under which Myer ob- 

 served the corona were exceptional. He watched 

 the eclipse from the summit of White Top Moun- 

 tain, near Abingdon, Virginia, 5,530 feet above 

 the sea-level. " The point of observation was 

 sought," he remarked, " with the view of placing 

 ourselves as far as possible above the lower and 

 denser strata of the atmosphere, and the smoke, 

 haze, and obstacles to vision with which they are 

 charged." As I remarked in the first edition of 

 my treatise on the sun, in 1870, the bearing of 

 General Myer's evidence on the question of the 

 effect which our own atmosphere produces on 

 the corona is specially important on this account. 

 I may add that, so far as the faint extensions of 

 the corona were concerned, it was in his favor 

 that he did not use a telescope except in the 

 study of the prominences. For it_ is quite a 

 mistake to suppose, as many do, that the appar- 

 ent highness of a luminous object is increased 

 •vhen a telescope is employed. 1 A telescope 



1 A telescope increases the quantity of light we 

 pjet from an object, supposing the whole object visible 

 in the telescopic field of view; and thus a telescope 

 increases the visibility of such an object. In the case 

 of a body like a star, which even in the most power- 

 ful telescope appears a mere point, the brightness is 

 increased precisely in the same degree that the visi- 

 bility or total quantity of light is increased. But the 

 case is very different with the brightness of a surface. 

 The moon seen through a telescope is not brighter 

 than the moon seen with the naked eye. She is not 

 even quite as bright. She looks much larger, and as 

 she thus presents a surface much larger and not 

 much less bright than when she is seen with the 

 naked eye, of course we get much more light from 

 her. In fact, the full moon, seen with a very large 



somewhat diminishes the brightness of any ob- 

 ject observed through it ; and, when the object 

 to be studied is large and of feeble lustre, the 

 telescopist has not so good a chance of detecting 

 it as one who seeks for it with the naked eye. 



It appears to me that the lesson conveyed by 

 General Myer's observation is, that to recognize 

 the faint extension of the corona a station as 

 high as possible above the sea-level should be 

 occupied, and that the naked eye should be used ; 

 or if a telescope, a small one only, with a large 

 field and low magnifying power. 



Before the eclipse of 1870 I suggested the 

 possible advantages to be derived from the care- 

 ful study of the corona with special reference to 

 the difficulties which arise from the extreme 

 faintness of the light of its exterior portions. In 

 the Monthly Notices of the Royal Astronomical 

 Society, March, 1870, I wrote as follows: "The 

 use of a telescope of low magnifying power but 

 first-rate definition would be desirable, a comet 

 eye-piece (that is, an eye-piece giving a large field 

 of view, as when faint comets are sought) being 



telescope on a clear night, sends to the eye an unbear- 

 able quantity of light, and can only be looked at for a 

 second or two without pain. But the large image is 

 unmistakably less bright than the small image formed 

 on the retina of the eye when the moon is looked at 

 without a telescope. It is easy to see that this is so 

 by looking with one eye through the telescope, and 

 with the other at the moon directly. It will be found 

 easy to bring the small, naked-eye image of the moon 

 close to the large telescopic image, when the superior 

 intrinsic brightness of the smaller image will be at 

 once recognized. It is sometimes urged that the pain 

 we experience if we look at the moon for any length 

 of time with a powerful telescope proves that the lunar 

 image must be much brighter ; for pain is never 

 caused by long-coutinued gazing on the moon with- 

 out a telescope. If, it is argued, the portion of the 

 retina on which the small, naked-eye image of the 

 moon is received experiences no inconvenience, how- 

 ever long the moon's image is allowed to remain 

 there, no inconvenience would be caused if another 

 neighboring part of the retina were occupied by a 

 similar imaire, a third part of the retina by another, 

 and so on, until as large a surface were occupied with 

 images of the moon as is occupied by the single image 

 of the moon seen with a powerful telescope : the 

 pain, then, actually experienced when this large image 

 is seen must he due to the greater intrinsic lustre of 

 the image. But, apart from the facts (1) that theoret- 

 ically the telescopic image must be fainter, and (2) 

 that the experiment before described shows it to be 

 fainter, the reasoning just described is altogether 

 faulty. We might as reasonably argue that, because 

 one hair can be plucked from the head without caus- 

 ing pain, a handful may be pulled away in equally 

 painless fashion; or that, because a man can endure 

 the pain of having one tooth extracted, he could 

 stand having a whole jawful taken out at one pull. 



