TWINKLING OF STARS. 



TY PR-FOUNDING. 



i .1 



Sinus presents Krttn, yellow, red. Mid blue repeatedly following in tbi 

 same order ' ;i alo in Ibl3 remarks that tlie circular duo u! 



aUr vacilUttx in such a manner u to give the idea of a number o 

 due* puling in succession before each other. Throe duo are o: 

 dilU-n nt colours : the light appeared to come from different aide*, auc 

 the mart frequent coloun were blue, steel blue, poagreen, brilliant 

 copper, red, and white. The same observer viewing Sirius throng 

 an achruuiatio by Ranwden, magnifying twenty-four timee, atid the 

 eye-piece adjusted fur distinct vision, the observer struck lightly 

 on the tube a aerie* of rapid blow* with his fingers, go as to uiako. 

 the image of the star describe a luminous line, every purt of w liirh 

 displayed the most lively coloun, and it was calculated that the light 

 of Sirius changed in colour before arriving at the eye ut least thirty 

 time* in a second. 



S. nitill.itu.ii preaenU phenomena too remarkable to have been allowed 

 to remain without attempU, at least, at explanation. Aristotle, who 

 noticed the scintillation of the fixed stars, and the steady light of 

 the planeU, apeaki of a want of fixity in our light for luch distant 

 objects, while the planeU being coinparativoly near can be viewed 

 with a steadier gaze. Ptolemy aUo refen to the trepidation nf the 

 organs of sight, and the consequent trembling of the heavenly body. 

 Alhazen and Vitellion refer the twinkling of star* to the effects of 

 refraction which the stellar rays experience in our atmosphere. Aguil- 

 lonius explains it by the rapid movement of rotation of the stars. 

 Tycho adopts such an explanation as when a diamond cut into facets 

 rotates, but the planets do not scintillate because he imagined them 

 not to rotate. Cardan adopts Aristotle's explanation. Scaliger, 

 among other attempts at explanation, refers to the intermittent light 

 of an incandescent body, and to the changes in the intensities of the 

 stellar light produced by vapours floating in the atmosphere. Galileo's 

 explanation assumes a vibration which stars impress upon their own 

 light. Kepler compares the stars to diamonds cut into facets so that 

 the least movement produces iridescent colours, or in other words 

 he supposed a star to contain angular surfaces unequally luminous. 

 Scheiner imagines scintillation to be an optical illusion. Descartes 

 explains the phenomenon by means of his favourite vortices or tour- 

 billons, which he supposed to surround all celestial bodies. Huyghens 

 explains scintillation by means of the vapours of the earth; and 

 Gasseodi attributes it to a vibratory motion in the eye. Riccioli calls 

 in the aid of atmospheric vapours, and imagines also that particles of 

 dust and opaque filaments floating in the air influence the phenomenon. 

 Hooke explains it by the irregular refractions of the stellar rays in our 

 atmosphere, arising from the unequal distribution of heat. Hit Isaac 

 Newton refers to the refractive power of the humours of the eye, and 

 also of the atmosphere, and speaks of a trembling movement in the 

 latter. Juriu endeavours to explain scintillation by means of Newton's 

 theory of fits of easy transmission and reflection. Cassini explains it 

 by supposing a sort of luminous coma to surround the stars, and to 

 undergo refraction and reflection in our atmosphere, but that these 

 scattered rays being united through the intervention of a telescope the 

 twinkling is less apparent. Long explains twinkling as the momentary 

 disappearance of the stars, in consequence of the interposition of 

 motes floating in the air. Mairan refers it to an undulatory move- 

 ment like that of the horizon seen over a vast plain illuminated by 

 the sun. Michell's explanation is based on the corpuscular theory of 

 light. Lalande refers to the agitation of the air, and to motes float- 

 ing in it, forgetting, as Long had done, that these motes must be at 

 least equal to the diameter of the pupil of the eye. Muschenbroeck 

 refen to the vivacity of light and to the activity with which it acts 

 on our organ of sight. Darwin explains twinkling by means of the 

 theory of accidental coloun which was much discussed in his day, and 

 explains it by the law of contrast. Saussure speaks of the oscillation 

 of the luminous rays produced by the alternate condensations and 

 dilatations of certain ports of the atmosphere. Dr. Thomas Young 

 says, " The cause of the twinkling of the stars is not fully ascertained, 

 but it U referred, with some probability, to changes which are per- 

 petually taking place in the atmosphere, and which affect its refractive 

 density. It is said that in some climates, where the air is remarkably 

 serene, the stars have scarcely any appearance of twinkling." Arago 

 remarks on the singularity of the fact, that the author of the doctrine 

 of interference of light, and indeed " of the only rea!;y new experi- 

 ment which has been made on scintillation from the timi. of Aristotle 

 to our own day, should not hesitate in facing the difficulties of the 

 problem to say, ' I do not know.' There is more true merit in this 

 candour than in the former unsatisfactory attempts at explanation." 

 Nicholson, however, was equally candid, for he professed his inability 

 to find in any of the known properties of light the cause of the phe- 

 nomena of twinkling. Biot in his ' Astronomie' bases his explanation 

 on the unequal refraction of the rays by our atmosphere. Forstur 

 imagines that the changes in colour which accompany twinkling must 

 be due to some change in the star itself, or that our atmosphere acts 

 like a prism on the rays of light. Cnpocci refen the phenomena to 

 our eye and not to any changes in the star, while Kamtz, in his 

 ' lieteorologie,' refer* to a su|>pnsod ottcilUtion of the star as a mere 

 point nf light about Its mean position, while a plane* having an :u 

 diameter of from SO to 40 seconds, it is more difficult to appreciate its 

 apparent change in volume. 



Lsntly we oome to the explanation of Arago himself. That keen 



observer was very much struck with the experiment of Nicholson, win. 

 observed Sirius through a telescope, and caused the image to \ 

 by tapping on the tube, when it appeared like a col., mi ,1 luminous 

 ribbon. On applying this experiment to other stars the to! 

 ribbon was observed with stars up to the sixth magnitude. hut no 

 trace of colour was observed with a star of the seventh. A 

 method of studying scintillation is to direct an achromatic telescope 

 towards a brilliant star and then to move the eyepiece out of focus. 

 The image then becomes an irregular disc approaching a ci: 

 and of a diameter greater or lew according to the position of tl.. 

 piece. The disc oscillates as though a number of discs 

 colours were in motion. A third method of observing 

 through ft U*lesco|M) is to contract the ajwrture by weans of an 

 opaque screen with a hole in the centre placed before the object gloss. 

 On viewing the image of a star out of tho focus, and con.v.|uently 

 enlarged, its centre will appear to be pierced with a dark but r 

 hole. The opaque screen used by M. Arago had a central opening of 

 only three or four centimetres in diameter ; the image of a star . 

 focus wax round and precise, but surrounded with a series of light 

 and dork rings perfectly well defined. The lustre of thise ilnj: 

 stantly varied in different parts of their contour, often disappearing, at 

 certain points entirely. Under these circumstances, if tl 

 were gradually pushed inwards, the image of the star would gradually 

 dilate, and a well defined black spot would appear in the centre. A 

 further motion of the eye-piece inwards caused the dark spot to 

 dilate into a small luminous disc, occupying the centre, surromule,! by 

 a dark ring, and this by a larger luminous ring. Or in a Uiii.i j- 

 of the eye-piece, still nearer to the object-glass, tin- centre of tho 

 image is obscure, but surrounded by a large brilliant i iug, sue, 

 by a dark ring, and this again surrounded by a luminous i . 



All these phenomena plainly refer the phenomena of scintillation to 

 the interference of light. [INTERFERENCE.] Luminous rays proceed- 

 ing from a point, such as a fixed star, and passing through our 

 sphere, subject as it is to great variations in heat, moisture, and 

 density, must have very unequal velocities j if such rays be brought 

 to a focus by means of a lens, as they are by the lens of the t \ 

 effect of their reaching us with different velocities will be seen by the 

 alternate brightening and darkening of the image in such focus, by :],. 

 known laws of interference. If, however, the rays proceed not i 

 point, but from a disc, such as a planet, the utt'ecU would so 1., 

 turb each other as to prevent their being observed, or if observed, l>ut 

 imperfectly. M. Arago has contrived three methods, or instrument 

 (friHtillomcten, as he calls them), based upon the change of the centnl 

 spot from dark to light, which occurs the more frequently in propor- 

 tion as the scintillation is strong ; for example, in preparing a tele- 

 scope as a scuitillonietcr as above directed, with a pierced creen 

 before the object-glass, on directing it to tho stars named in the lust 

 column of the following table, at the heights above the 1, 

 indicated in the second column, the central point became luminous, in 

 the course of five minutes, the number of times mentioned in tho 

 third column : 



January M, 1BS1. 



Sirins 10 40 



Kllfrt 81 17 



Aldebnran J7 3 IS 



LsChevre 81 S 



By observations of this kind M. Arago thinks it possible to decide 

 what are the climates, the seasons, the elevations, and other circum- 

 stances, under which scintillation entirely disappears. 



While writing this article, we have received from M. Andres Poey, 

 Director of the Pbysico-Meteorological Observatory of Uavannah, a 

 Memoir entitled ' Loi de la Coloration et do la Decoloration dca 

 Etoiles, du Soleil, et des Planetes,' reprinted from the ' Aunuaire ' of the 

 Meteorological Society of France, vol. viil, 1801. Our space will not 

 allow us to do more than point out one remarkable result obtained l>y 

 il. Poey, namely, that when, in viewing a fixed star through a telescope, 

 the eye-piece is drawn out the series of rings obtained ore comple- 

 mentary in colour to those seen when the eye-piece is push 

 awards the object-glass. 



TYCUE, in the Greek mythology, one of the goddesses of I 'i-.-tiny : 

 with the Komans FOHTCNA was ncar}y the correspondent deity, but 

 uniting in herself the attributes of the Greek goddesses of distiiiy was 

 on the whole of a somewhat st< 



daughter of Zeus tho liberator. She was the gnidcr of the attain of 

 ,he world, the bestower of good fortune, the unpcnso; m.v, 



uncertainty, and transitoriness. In art, on gems, coins, Ac., xh< is 

 frequently represented with attributes indicative of one or oil 

 ihoso qualities; with the rudder as providence; tho win , 1 o: ball as 

 chance and uncertainty ; with the horn ol plenty, or with a put< 1.1 in 

 Jio right hand, and ears of wheat aud poppies iu the left, as the 

 MDtower of gifts; as the good-fortune of cities she is figured as a 

 richly draped female with a crown of towers, a cornucopia and other 

 attribute of prosperity. Tyohe hod temples devoted to har wonUa 

 At Smyrna, at 1'borae in Messania, at yKgiria in Achaia, at '1 

 elsewhere. (Zoega, ' Ty lie mid Nemesis ; \ MuUer, 'Arcu.i, , 

 Kunst.' ami ' 1 1. A Hen Kunst.') 



TYPK-FO UN DIN G. [PWHTUtO.] 



