Necker, L. A. Esq. (1832). LXI. Observations on some remarkable phænomena seen in Switzerland; and an optical phenomenon which occurs on viewing of a crystal or geometrical solid. Philosophical Magazine, 3, 329.

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THE LONDON AND EDINBURGH

PHILOSOPHICAL MAGAZINE

JOURNAL OF SCIENCE.

[THIRD SERIES.]

NOVEMBER 1832.

My Dear Sir,

I must not delay any further fulfilling my promise of writing to you, respecting the subjects of our conversation in the too short moments I had the very great pleasure of good fortune of seeing you in London. I am the more bound to write soon, as my first object must be to correct a wrong statement in the date of my observation of the perihelia, as stated in my letter to Mr. Forbes (Edinb. Journal of Science, No. 12. p. 251.). As this mistake, into which I was led by trusting too much to my memory, and to some wrong inferences, has had the effect of weakening some circumstances which were in favor of your explanation, I am the more anxious to do it all justice, so that you may be in time to announce in your next Number the consequences of this mistake, for which I beg leave to apologize to you, to Mr. Forbes, and to your readers, whom I have unwillingly led into error. The true date of the day when I saw the perihelia, was the 1st of June 1830, as I observe by the little memorandum I kept of this remarkable, and to me entirely new phenomenon,-and not the middle of July, as I stated in my letter, written from memory in Edinburgh. Here I transcribe the whole nole, together with the little sketch which it contains.

Parhelia seen the 1st of June 1830, from 5½ o’clock P,M. till sunset at 7¼ o’clock, represented in the most complete state, as I saw it at 6 o’clock :-Sis thesun; EADC the inner halo, which was much the brightest; C and E the two lu minous images or mock suns; i’G the outer halo, which was weak, and seen only for a few mome11ts, as well as the inverted arch H. All the various points of these arches were not equally distinct at the same moment as they are represented here. On the contrary, when the image or mock sun at the right hand, C, was strong, the one at the left hand, E, was pale, or did not appear; such was the case at the beginning of the apparition. At the end, the left image, E, was very luminous and colored, shining with prismatic colors; while the 1·ight image C was less visible, and sometimes altogether wanting. The st1·ength of illumination of the various parts of the halo was constantly variable. Often certain portions, sometimes very considerable, entirely disappeared, and af terwards reappeared again. A little before the sun had set, the only part visible was that between A and B, and it was vividly lighted and coloured, and reflected by the lake ; at the same time the single point C was also shining brilliantly, colonre<l with iridescent colours. At the moment of sunset, ihere remained nothing but a small arch in D. The ph::e nom enon ended almost immediately after the sun had disap peared bel1ind the Jura at 7h 20m. All this time the parts of the sky situated to the west and north-west were hazy, and with some little clouds; while the eastern and southern pal’ts were perfectly pure and clear, and the chain of the Alps quite pure and bright.

The very rough and inaccurate sketch is a copy of the one which I made rapidly at the time, to preserve the memory of the fact. I well know that the halos and arches must be por tions of perfect circles, and parallel to each other ; but it be ing easy to make this col’rection in the mind, I preferred giving the thing as I sketched it in haste two years ago.

I am happy now to be able to give accurate information about the state of the atmosphere in tl1e day itself of the phrenomenon, and in thedays preceding it, by referring to the meteorological tables of the Bibliotheque Universelle, to which you may look for more particular details. I see that on the 24th of May 1830, the thermometer of Reaumur had stood be tween 10° minimum and 20°·8 maximum; then came rain; andby my notes I see that snowfell on the 25th on the Jura, which was melted on the 26th. On theSt.Bernard (1278 toises above the level of the sea), the 24th of May, the temperature was between + 2° R. minimum, and + 8° R. maximum, when rain fell, and the thermometer on the 25th of May descended to -0°·2 R. minimum, and + 4°·5 R. maximum. On the 27th and 28th of May, snow feJl on the St. Bernard, and the tem perature decreased till the 29th of May, when it was so low as to reach – 6°·1 R. minimum, and +4°·5 maximum. On the 30th of May it hadr isen again to – 4°·8 R. minimum, and + 5°·4 maximum; and on the 31st to – 1°·1 minimum, and + 5°·7 maximum : so much for the temperature of the l1igh parts of the atmosphere at the St. Bernard. During the same time, in the lowland at Geneva, since the rain of the 25th of May, the thermometer had gradually lowered till the 30th of May, when it had attained + 3°·2 R. minimum, and + 15°·4 R. maximum. On the 31st of May it had already risen to +10° R. minimum, and + 14°·4 R. maximum. Now on the 1st of June 1830, the day of the parhelia, the thermometer at Geneva was between + 3°·5 R. minimum, and + 17°·3 R. maximum. The last must have been nearly the tem perature during tbe phamomenon in the plain. .At the St. Bernard on the same day, the thermometer was between – 3°·6 >minimum and + 9° R. maximum. This last temperature may give an idea ofthatoftbe atmospheric strata at l000toises above Geneva, at the time of the The whole day was serene and cloudless at the St. Bernard. At Geneva it was likewise so, except in the afternoon, when a thin mist or ha , audsome light clouds, appeared in the west.

From the combination of all these circumstances, it remains not unlikely, nay, even probable, that icy particles may have been floating in those light mists which gave rise to the par)1elia,if we suppose their height to exceed a good deal that of 1000 toises above Geneva, or 1280 toises above the level of the sea.

Although mistaken in my former statement of the epoch on which the parbelia took place, considering that June, though not the hottest, as I said of July, is at least a hot month of our summer, that the occurrence of a parhelion in that season, and in such a latitude as ours (46″ 121 N. lat.), is a very rare thing, and that by the knowledge we have been able to get of the meteorological circumstances attending such a phamo menon (circumstances whicl1I <lo not believe have been men tioned in similar accounts of parhelia),-we are able to form an idea at least of the minimum of height nt which the refracting medium causing the parhelia must have been placed. I do not regret to ba\·e drawn your attention to this fact, which, instead of militating against, will rather tend to corroborate your ideas as to the necessity of supposing minute crystals of ice to explain the phenomenon.

I now come to the point which you particularly wished me to describe to you : I mean the laminous appearance of trees, shrubs and birds when seen from the foot of a mountain, a little before sun-rise. The wish I had to see again the phre- nomenon before attempting to describe it, made me detain this letter, a few days, till I had a fine day to go to see it at the Mont Saleve; so yesterday I went there and·studied the fact, in elucidation of which I made a little drawing, of which I give you here a copy: it will, with the explanation and the annexed diagram, impart to you,1 hope,a correct idea of the phrenome non. You must conceive the observer placed at the foot of a hill interposed between him and the place where the sun is rising, and thusentirely in the shade; the upper margin of the mountain is covered with woods, or detached trees and shrubs, which are projected as dark objects upon a very bright and clear sky, except at the very place where the sun is just going to rise; for there all the trees and shrubs bordering the mar-• gin are entirely, branches, leaves, stem, and all, of a pure and brilliant white, appearing extremely bright and luminous, although projected on a most brilliant and luminous sky, as that part ofit which surrounds the sun always is. All the mi nutest detail , leaves, twigs, &c. are most delicately preserved,and you would fancy you saw these trees and forests made of the purest silver, with all the skill of tl1e most expert work man. The swallows and other birds flying in those particular spots appear like sparks of the most brilliant white*. Unfortunately all these details, which add so much to the beauty of this splendid phamomenon, cannot be represented in such small sketches. Neither tl1e hom of the day,nor the aiigle which the object makes with the observer, appears to have any effect; for on some occasions I have seen the phamomenon to take place at a very early hour in the morning. Yesterday it was 10 o’clock _A,M., when I saw it as represented in .fig . 1.

I saw it again on the same day at 5 o’clock P ,M ., at a different place of the same mountain, for which the sun wasjust setting. At one time the angle of elevation of the lighted white shrubs above the bori:mn of the spectator was about 20 ° ; while at another place it was only 15°. But the extent of the field illuminated i;; vtU’iable, according to the distance at which the spectator is placed from it. When the object behind which the sup is going to rise, or has just been setting, is very near, no such effect takes place. In the case represented, fig. I, the distance was about 194 metres, or 636 E nglish feet, from the spectator, in a direct line; the height above his level being 60 metres, or 197English feet,and the horizontal line drawn from him to the horizontal projection of these points on the plane of his horizon being 160 metres, or 525 English feet, as will be seen in the following diagram, fig. 3. 1n this case only small

[* This appearance seems to be connected with that asmmed by flying birds when seen, under certain circumstances, through a telescope, during obserrntion s on the sun, and which, it has been allege,!, has occasionally been mistaken for that of small met eors seen in the day-time : see the next two pages.-EolT,)
shrubs, and the lower half of the stem of a tree, are illumina… ted white, and the horizontal extent of this effect is also com• paratively small; while at other places when I was nearer the edge behind which the sun was going to rise, no such effect took place. But, on the contrary, when I have witnessed the phamomenon al a greater distance and at a greater height, as I have seen it other times in the same and in other mountains of the Alps, large tracts of forests and immense spruce firs were illuminated white throughout their whole leng th, as I have attempted to represent in fig. 2. and the corresponding diagram, fig. 4. Nothing can be finer than these silver-looking spruce forests. At the same time, though at a distance of more than a thousand metres, a vast number oflarge swallows or swifts (Cypselus alpinus), who inhabit those high rocks, were seen in the shape of small brilliant stars or sparks moving rapidly in the air. From these facts, it appears to me obvious that the extent of the illuminated spots varies in a direct ratio of their distance; but at the same time that there must be a constant angular space, corresponding, probably, to the zone, a few minutes of ₁₁degree wide, around the sun’sdisk, which is a limit to the occurrence of the appearance: this would explain how the real extent which it occupies on the earth’s surface varies with the relative distanceof the spot from the eye of the observer, and accounts also for the phrenome non being never seen in the low country, where I have often looked for it in vain. Now that you are acquainted with the circumstances of the fact, I have no doubt that you will easily observe it in some part or other of your Scotch hills: it m y be, some long heaths or !urze will play the part of our lpi e forests; and I would advise you to try to place a bee h1ve m the required position, and it would perfectly represent our swallows, sparks or stars.
I now only wonder that such a phrenomenon, which must necessarily take place in every mountain of the earth, every day and at every hour of the day when the sun shines, should never have been noticed before.

I now come to another subject which you also desired me to mention: I mean the varying colours exhibited by Mont Blanc during and after sunset. Lord Minto was perfectly right in the account he gave you of these appearances. But he may have omitted some circumstances which will assist in leadina us to an explanation of tl1ese Yarying appearances. I shalf here state the facts in the order in which they appear. When the sun is near setting, and the weather is serene, all the mountains of the Alps, facing the west, are tinged with a fine purplish hue, which on Mont Blanc, on account of its bright covering of snow, takes a tinge more verging towards a light orange. When the sun has set for the plain, these mountains appear more vivid and more illuminated, by the effect of contrast. When, some minutes after, the lower mountains are in the shade, their purple hue is changed into a <lark blueish tinge, the contrast between their shaded parts and those that we1·e lighted by the sun has disappeared, an<l an almost uniform grayish blue shade covers them all; at this time Mont Blanc remains the only terrestrial object still lighted by the rays of the sun, and that circumstance causes its immense mass of snow to appear more bright, and its yel lowish orange colour more vivid: at the same time the con trast between the projected and other shadows and the lighted parts is at its maximum (I have two or three times seen Mont Blanc at that moment, and when dark clouds were be hind it, look almost as bright and red as a live coal). When, however, the sun has set for Mont Blanc, which happens about a quarter of an hour after it has set for the plain round Geneva, then the whole ofMontBlanc assumes a dull blueish white hue, and a flattened appearance, owing to the absence of contrast from the once shaded parts with those that were lighted. And so its new aspect is to that which it offered a few minutes before, like that of a dead body to a living and healthy one. This pale and, as it were, morbid appearance of the mountain is owing to the fact, that above it exists still a wide zone of atmosphe1·e loaded with thin and light vapours, for which the sun has not yet set, and which on that account are still lighted vividly, and coloured with. a purple hue. \Vhen, however, the sun has also been setting for these higher re gions of the atmosphere, the contrast between their illumina tion and the shade existing all over Mont Blanc, to which was owing that blueish and deadly colour assumed by the eternal snows, having ceased to take place, the Mont Blanc assumes once more, but in a much fainter and<larker manner, its orange yellow colour; and the lower and nearer moun tains recover their purplish hue. All the objects then being uniformly and altogether illuminated by the much paler ancl less powerfol light of the twilight, as they were before all lighted at once by the brighter, but equally uniformly spread, light of the sun; so that every thing being placed in the same relative quantity and quality of illumination as before, an analogous aspect is seen in both cases, though much darker under the latter than under the former circumstances. Hence it appears to me that the whole of the phrenomenon is most naturally and easily explained by contrast.

The object I have now to call your attention to, is :m ob servation which is also of an optical nature, and which has often occurred to me while examining figures and engraved plates of crystalline forms: 1 mean a sudden and involuntary change in the apparent position of a crystal or solid repl’e sented in an eno-raved figure. What I mean will be more easily understood from the figure annexed. The rhomboid AX is drawn so that the solid angle A should be seen the nearest to the spectator, and the solid angle X the furthest from him, and that the face ACBD should be the foremost, while the face XDC is behind. But in looking repeatedly at the same figure, you will perceive that at times the apparent position of the rhomboid is so changed that the solid angle X will appear the nearest, and the solid angle A the furthest; and th/lt the face ACDB will recede behind the face XDC, which will come forward ; which effect gives to the whole-solid a quite con trary apparent inclination. I have been a long ti.me at a loss to understand the reason of the apparently accidental and involuntary change which I always witnessed in all sorts of forms in books of crystallography. The on\y thing I could observe was, that at the time the change took place, a parti cular sensation was felt in the eye (for it takes place as well when seen with only one eye, as with both eyes), which proved to me that it was an optical, and not merely as I had at first thought a mental, operation which was performed. After, how ever, e. more attentive analysis of the fact, it occurred to me, that it was owing to an involuntary change in the adjustment of the eye for obtaining distinct vision. And that whenever the point of distinct vision on the retina was directed on the angle A, for instance, this angle seen more distinctly than the others was naturally supposed to be nearer and fore most; while the other angles seen indistinctly were supposed to be further, and behind. The reverse took place when the point of distinct vision was brought to bear upon the angle X. This solution being found, I proved that it was the real one by three different ways. 1st, By being able at my will to see the solid in which p·osi tion I chose, and to make this position vary at pleasure, in looking alternately, with fixed attention, either to the angle A, or to the angle X. 2ndly, ,vhile looking steadfastly to the angle A, ans} seeing the rhomboid in its proper position with the angle A fore most, if without moving either the eye or the figure, I made a convex lens (such as is used in spectacles for long-sighled ness,) pass gently from below upwards between the eye and the figure, at the instant when the figure was visible through the glass, the change had taken place, and the solid had assu med the apparent position in which the angle X was the fore most, and that only because, owing to the refraction through the glass, the image of the angle X had come to take the place of the real angle A, and so the point of distinct vision, with out being at all moved, had by this means come to bear on the angle X, or rather on its image.

3rdly, If through a hole made with a pin in a card you look at the figure in such a manner that either the angle A or the angle X be hidden, the visible angle will determine the appa rent position of the solid, so that this angle will always appear the nearest; it will he impossible to see it in any other way, and consequently there will be no change.

What I have said of the solid angles is equally true of the edges,-those e<lges upon which the axis of the eye or the central hole of the retina are directed will always appear forward so that now it appears to me certain that this little, at first so puzzling, phenomenon, depends upon the law of distinct vision.

You surely will draw from all the above communications, many consequences which my ignorance of the subject prevents me from anticipating. You may do what you think most proper with all these observations.

I remain, my dear Sir, with the kindest regard,

Ever most sincerely yours,

Geneva, May 24, 1832. L. A. NECKER.