457 



TWILIGHT. 



TWINKLING OF STARS. 



453 



tables became obscure, and this, with the great change of circumstances' 

 must have rendered many of their provisions inapplicable. In the 

 later times of the republic, Cicero observes that since his boyhood the 

 practice of learning the Twelve Tables had been superseded by the 

 growing importance of the Edict. 



The Roman jurists made commentaries on the Twelve Tables. Six 

 books of a commentary by Gaius are mentioned, which shows that at 

 least as late as the time of Antoninus Pius the decemviral law was in 

 substance still in force, that is, that the fundamental principles of 

 Eoman law (the Jus Privatum) were still to be sought in the then 

 antiquated language of the Decemviri. 



Much has been written on the subject of the Twelve Tables. The 

 last and most complete history of the labours of modern critics on the 

 Twelve Tables is by Dirksen, in his ' Uebersicht der bisherigen Ver- 

 suche zur Kritik und Herstellung des Textes der Zwblf-tafel Frag- 

 ment*,' Leipzig, 1824. The fragments of the Twelve Tables appear in 

 the several parts of the work, and are also collected at the end. 



For an excellent dissertation upon the Twelve Tables as illustrative ' 

 of the archaic form of the Latin language, the reader is referred to an 

 able article in the late Dr. Donaldson's ' Varronianus.' 



TWILIGHT, the name given to the light which remains after the 

 sun has set, or which is seen immediately before it rises. The reason 

 of this appearance is explained in SUN (col. 915), being the effect of the 

 light which is reflected from the higher strata of the atmosphere, in a 

 manner which will be understood from the diagram in the column 

 cited. 



In our latitudes, at the summer solstice, a portion of the twilight 

 continues from the setting of the sun to its rising, circling, as the 

 hours of night and morning proceed, from the western horizon through 

 the north, to the east ; which is the cause of there being scarcely any 

 true night at that period of the year. 



J. H. Lambert endeavoured to distinguish, besides the primary 

 twilight, a secondary and even a ternary twilight, both the latter being 

 caused by the successive reflection, by the clouds and the air, of light 

 already reflected from other regions of the atmosphere, their clouds, 

 4c. In conformity with these views, Sir John F. W. Herschel has 

 attributed to such a cause the phenomenon, seen in the clear atmo- 

 sphere of the Nubian deserts, which has been described by travellers 

 under the name of the " after-glow." To a corresponding cause must 

 be ascribed the rose-coloured illumination of the summits of high 

 mountains after sunset (often witnessed of those of Mont Blanc and 

 Monte Rosa), but in these cases the reflected light is coloured by its 

 traversing tracts of the air of which the vapour is in an opalescent 

 state, imparting to it various hues of red and orange. [VAPOUB.] 



The astronomer last named, always careful to explain the physical 

 processes which are operative in the production of astronomical pheno- 

 mena, in addition to the mathematical principles on which they depend, 

 has pointed out that a portion of the light of the sun and moon reaches 

 us after they are set, by means of the atmosphere, " by reflection upon 

 [and from] the vapours and minute solid particles which float in it, 

 and, perhaps, also on the actual material atoms of the air itself." (See 

 ' Outlines of Astronomy.' ed. 1858, par. 44, 45.) 



The observations which have been made during the last two or three 

 total solar eclipses are very instructive, in two respects, concerning the 

 light ordinarily received by the earth, including that of twilight. The 

 character of the darkness, while the totality continues, so much more 

 intense than that of night caused by the mere aversion of the hemi- 

 sphere from the sun, evinces how intrinsically dependent upon that 

 luminary for light the earth really is ; while the amount and pecu- 

 liarity of the illumination actually existing at the same time, shows in 

 how great a degree we are indebted to the reflective and refractive 

 powers of the atmosphere, and to the reflection by the floating particles 

 alluded to, by aqueous vapour becoming visible but not yet cloud, by 

 the clouds themselves, and by the earth, reciprocally and unitedly, for 

 the light we enjoy after the commencement and during the progress of 

 that aversion. 



But all this is, of course, affected by the degree of transparency of 

 the atmosphere and its difference at different altitudes, the less trans- 

 parent, in the mass, so far as we know, being those nearest the earth ; 

 while there are facts which indicate that air perfectly free from aqueous 

 vapour, such as we must conclude it to be above a certain height, is 

 less transparent, or more absorptive of light, than the mingled atmo- 

 sphere of air and aqueous vapour incumbent on the earth's surface, 

 when the temperature is such as to sustain the latter in a perfectly 

 gaseous condition. When the sky is thus free from visible vapour 

 and cloud, however, the transparency of the atmosphere is almost 

 invariable, as Professor Seidel has shown. 



Another consideration affecting this subject relates to the probable 

 nature of the highest regions of the atmosphere, on which the amount 

 of reflected light causative of twilight must be greatly dependent. If, 

 as inferred by Graham and Poisson [SURFACE OF THK EARTH, col. 932], 

 the terminal stratum be solid air-ice the reflection from its inferior 

 surface, and from the inferior surfaces of the comparatively dense 

 strata immediately below it, must be more powerful than that from 

 the rare, purely aeriform strata (of which alone the highest regions are 

 commonly supposed to consist), and hence may exert a marked effect 

 in prolonging the twilight, as well as in lessening the darkness during 

 the entire time when the sun is below the horizon. It may be that 



this reflection is the principal agent in producing the remarkable 

 amount of light still remaining during a total eclipse of the sun, as 

 noticed above. - What may be the bearing upon this subject of the 

 observed polarising power of the sky during such eclipses has not 

 been investigated, though the materials for such an inquiry, we believe, 

 have been obtained, especially from the eclipse of July 18th, 1880. It 

 seems probable that, the polariseope would furnish the means of 

 determining whether such reflection does in reality take place, and also, 

 if taken in conjunction with the atmospheric refraction of the heavenly 

 bodies, of determining what the structure and constitution of the 

 upper regions of the atmosphere really are. 



Intimately connected with this particular subject is the application 

 of the phenomena of twilight, or rather of the amount of depression of 

 the sun at the close of twilight, to determine the height of the atmo- 

 sphere, first proposed by Kepler, and which, iu fact, gives a height 

 nearly agreeing with that inferred from the law of elasticity of the 

 air, of between forty and forty-five miles. But the argument from the 

 observed depression of the sun is inconclusive, because we do not 

 know when twilight has ceased, nor, indeed, whether it ever ceases. 

 According to Leslie, admitting, from the fact that in clear weather in 

 no climate is there total darkness, even at midnight, '' that the body of 

 air extends to such an altitude, as to receive the most dilute glimmer, 

 after the sun has attained his utmost obliquity, and sunk ninety 

 degrees below the horizon," the elevation of the atmosphere must be 

 equal at least to Iti38 miles. In this reasoning, however, no account is 

 taken of the necessary limitation of the atmosphere by the cold of 

 interplanetary space, which must have effect at a point greatly nearer 

 the earth. On the other hand, the actual demonstration of the ex- 

 istence of a solid or liquid stratum at the summit of the atmosphere, 

 by the means suggested above, would probably involve also, the deter- 

 mination of its altitude, and, conversely would enable us to fix, 

 definitively, the extent of twilight upon the earth's surface. [ATMO- 

 SVHEUE; METEOROLOGY.] 



A question here arises as to the photographic iutensity and proper- 

 ties of the reflected light from which the phenomenon of twilight 

 proceeds, as compared with those of the direct sun-light itself. But 

 this comparison does not appear to have been made iu any express 

 manner. According to the photo-chemical researches of Buuseu ami 

 Roscoe ('Phil. Trans.' 1859, p. 898), the chemical illumination, that is, 

 the affection by the chemical rays, of the earth's surface, is merely a 

 function of the sun's zenith-distance ; all the elements of the sun's 

 radiation (so to call them), light, heat, and chemical action, diminishing 

 equally with his altitude. As the diffused light of day is all, in its 

 immediate origin, reflected, and as it, in common with that reflected 

 from the clouds, possesses normal chemical action, differing only in its 

 intensity from that of the direct light of the sun, we may infer that 

 a similar difference only exists iu the case of twilight. 



TWINKLING OF STARS, or SCINTILLATION (scintilla, "a 

 spark of fire"). This term is applied to a phenomenon which has 

 attracted the attention of astronomers and scientific men in all ages. 

 It consists in rapid variations iu the brightness of a fixed star when 

 observed with the naked eye, and is often accompanied by changes in 

 the colour, and alterations iu the apparent diameter of the star or in 

 the length of the diverging rays which appear to dart from its centre 

 in different directions. It is commonly stated that the twinkling 

 disappears when the star is viewed through a telescope ; such, however, 

 is not the case, although under such circumstances the phenomena are 

 modified. 



This subject occupied but a very small space in scientific works, if 

 indeed it was to be found at all in them, until M. Arago devoted one 

 of his searching scientific notices to the subject in the ' Aunuaire pour 

 1'An 1852, publi6 par le Bureau des Longitudes;' nevertheless the 

 importance of the subject has never been forgotten, and Kepler even 

 invited scientific men to a conference on the subject, and appointed 

 Frankfort as the place of rendezvous. 



The changes in colour which accompany scintillation, and form one 

 of its most important features, were noticed by early observers. Indeed 

 the name given by the Arabs to Sirius refers to this fact ; they call it 

 larakesc/i, or " the thousand-coloured star." Tycho, writing in 1572, 

 respecting the new star of that year, compares it to the reflections of a 

 cut diamond moving in the presence of light. Kepler also refers to 

 the Dog Star as presenting by turns all the colours of the rainbow. 

 Hooke, in his ' Micrographia,' refers to the various colours which 

 accompany the scintillation of stars, appearing red at one moment, 

 yellow at another, and blue at a third. Forster (' PhiL Mag.,' 1J-JJ1) 

 remarks that sometimes the intensely red light appears after two 

 dilatations of the star, under other circumstances after three, but 

 ofteu without any apparently regular law. Several observers notice 

 the scintillation of the planets; but no astronomer refers to their 

 change of colour. Scintillation in their case is a simple change in the 

 intensity of the light. 



The scintillation of a star, when viewed through a telescope, was 

 first described by Simon Marius, who recommends that the eyepiece 

 be removed from the telescope, and the eye be substituted for it, at a 

 time when the sky is very clear and the air tranquil. The scintillation 

 will then appear like a fulmiuation or ebullition of the substance of 

 the star, and certain determinate and distinct colours will appear in 

 greater or less abundance according to the stars observed. Thus 



