1862.] 



ON THE ATMOSPHERIC PHENOMENA OF LlOHT. 



axis biiing ouo-uiglitli greater Cluiii the vertical. Various tallies 

 have been constructed tor gi\ing tlie aniouut of correction to be 

 applied at different altitudes : the best English ones are constructed 

 from the following expression which is to be deducted from the 

 obser\ed zenith-distance. 



c.tan.z (1 — .00128 sec^z) 

 Where z is the apparent zenith-distance, and c represents the 

 variable tpantity 6'J"6f) h. in which a; is the tem- 



r+.UU2USU3j;' 29.93 

 perature of the air expressed by the number of degTees above 

 freezing point, (Fahrenheit) and /* the height of mercury iu the 

 barometer in inches. This formula is obtaiiird iii;l(.'|«'ndcn(ly of 

 any hypothesis as to the law of variation of (leii>ily, only ;i^-muing 

 that the density is the same at ecpal distuuees tioiu llir Earlii's 

 centre ; and it is sufficiently accurate for astronomical purposes for 

 all altitudes above 26°, but below this the law of variation must be 

 taken into account, and as we are altogether ignorant of this law, 

 the foriuulie and tables for low altitudes are more or less empirical. 

 A remarkable and ingenious one was constructed by Laplace 

 (Mec. Cel.,) and the French tables are founded upon it : but very 

 near the horizon, the irregularities of refraction, arising from local 

 and accidental circumstances, are so great as to foil all attempts to 

 express them by a mathematical formula. 



It is a singular fact that the changes of humidity in the atmos- 

 jihere do not produce any sensible effect on the refraction ; the 

 reason being that the density of suspended vapour is less than that 

 of air very nearly in the same ratio that its refractive power is 

 greater, so that the effective refractive power of aqueous vapour is 

 about the same as that of the atmosphere. The effects produced 

 by refraction are sometimes exceedmgly curious. When the stra- 

 tum of air next the Earth differs very much in density from that 

 above it, the rays from an object which would not otherwise reach 

 the spectator, may be bent back from the higher stratum and thus 

 furnish an image in addition to the one seen by direct rays, and 

 elevated or depressed with regard to it according as the higher 

 medium is rarer or denser than the lower one. Thus if the tem- 

 perature of the sea is higher than that of the atmosphei'e, owing to 

 the slower coolmg of the former, the stratum of air immediately 

 above the sea becomes rarer than the one higher uj) : and a specta- 

 tor situated iu the denser medium and looking at an object in that 

 stratum, mil see, at the same time with the image furnished by 

 direct vision and below it, an inverted image produced by rays bent 

 upwards from the lower medium. To this class of phenomena 

 belong the well-known Fata Morgana, the appearances seen on 

 the sandy plains of Egypt, and called by the French Mirage, and 

 the Looming occasionally seen in parts of Great Britain. 



When the higher stratum is rarer than the lower, as sometimes 

 liappens from the air above the sea being suddenly heated by the 

 Sun, the appearances wUl be reversed, an inverted image being- 

 formed as before, but in this case elevated above the true image. 

 A second sudden change of density in the ascending strata may 

 give rise to another image still, elevated above the other two and 

 erect, as actually observed by Cap)tain Scoresby, who.-ilsn ivlntes 

 an instance in which he recognized his fatlii'i's \r>s.'l, w Inn nt a 

 distance of 30 miles, and therefore far beyond the limit of direct 

 vision, by means of an inverted image in the air, so well defined 

 that every sail could be distinguished. This also ])oints to the 

 probable explanation of the remarkable case of "the French sailor 

 in the Mauritius, who was accustomed to pi'edict the approach of 

 vessels long before they could be detected by the t^escope, and 

 wdien they must have been far below the horizon. 



Phenomena of a somewhat similar nature are sometimes produ- 

 ced by reflection, a slight haze acting as a mirror. The spectre of 

 the Brocken is well known, and a beautiful instance is recorded as 

 having been seen by Dr. Buchan at Brighton, (see Sir D. Brewster's 

 Natural Magic.) The same has been obser\-cd at the Mountain- 



house on the Catskills, when at sunrise the face of the cliff' and the 

 front of the house were seen by the spectators standing on the ledge 

 on which the house is built, together with their own images, vividly- 

 reflected in front of them as the morning mist just cleared away.* 



The aqueous particles suspended in the air as clouds, or fiiUing 

 in the shape of rain or snow present many beautiful phenomena. 



The gorgeous colouring of the clouds with their infinite vaiieties 

 of hght and shade are readily explainable on the principles above 

 indicated, the Sun's light Ijeing transmitted in different tints 

 according to the depth and density of the vapour through which it 

 passes, and the clouds themselves reflecting or transmitting it 

 variously according to the position they occupy relatively to the 

 Sun and spectator. In the morning and evening, the clouds, float- 

 ing with their largest dimensions horizontal, present greater masses 

 of vapour to be traversed by the Sun's rays than when he is vortical, 

 and this, along with the circumstance of the longer horizontal range 

 of the air, gives the reason why the sky is then distinguished by 

 richer colouring than at mid-day. 



Among the arrangements of clouds which produce remarkable 

 effects may be noticed those of diveig'ing and converging beams, 

 known in the country by the name of '• the Sun drawing water;" 

 they generally indicate wet weather, as they only occur when the 

 air is charged with vapour. The former arises simply from some 

 of the Sun's rays being stopped by clouds whUe others are aUowed 

 to pass through openings therein ; or, to speak more con-ectly, from 

 the shadows of clouds being projected on the sky so as to obscure 

 parts of it in the neighbourhood of the Sun ; it most frequently 

 occui-s when the Sun is not far from the horizon. The latter phe- 

 nomenon is of much rarer occurrence, and consists of arcs of great 

 circles apparently cutting each other in a pioint of the heavens 

 below the horizon in the prolongation of the fine drawn from the 

 Sun to the spectator. This apparent convergence is pm-ely an affect 

 of perspective, the rays themselves being parallel but appearing to 

 approach each other as their distance from the spectator increases, 

 hke the opposite rows of gas-lamps in a long street. Another 

 singular form is that to which the French have given the name of 

 bandes polaires, when the arrangement of the clouds is such as to 

 cause the illmninated portion of the sky seen between them to 

 assume the form of an auroral arch, but considerable obscurity still 

 hangs over this phenomenon. 



When a light cloud is intei-posed before the Sun or Moon, the 

 disc is often surrounded by several coloured rings, each displaymg 

 the prismatic tints with the red on the inside and the violet out- 

 side, the diameter of the inner ring varying from 1 ^ to 4 degrees : 

 these rings are called corona, and are most commonly seen round 

 the Moon; wdien round the Sun, they are best discerned by 

 reflection in water, and in this way Newton succeeded in detect- 

 ing three at once. The cause of this appearance baffled even 

 Newton's sagacity, and it was reserved for the famous Young to 

 point out, that the rings resulted from his doctrine of the inter- 

 ference of rays on the undulatory hypothesis, the same cause that 

 produces the colours of a soap bubble and the prismatic tints of a 

 spider's web. Young announced from theory that the diametere 

 of the successive rings would be as the natural numbers 1, 2, 3, 

 5, - - - and this prediction has been verified by the obser- 

 vations of M. Delegenne. The same principle explains also a 

 phenomenon, first noticed by Bouguer, and called by him anthelie, 

 that when the shadow of a spectator is thrown on a cloud, or on 

 the ground covered with dew, the shadow of the head is seen 

 surrounded with coloured ring's, like the "glories" round the 

 heads of saints in old ]Mctures : a similar effect has been observed 

 by M. Babinet when his shadow fell on the smoke of artillery, 



*I cannot refrain frotii rccoiiimendinjj all lovers of sceneiy and science 

 lo pay a visi; lo llit; Mnujiluin House al Calskill ; llie view of sunrise from 

 tlie plaKonn, and llie irises of llie walerfall om-rni. 

 ances, and liie njlural bcaulies of llie neiglibourh 

 by the fad of the liulel being an txcellunl one. 



. ns oplieal appear- 



ud are not delracled from 



