July 25, 1872] 



NATURE 



253 



inclined to think it must lie to the eclipse of August, 1 133, that 

 the record applies, though previous or subsequent events may 

 have been mixed up with it by the historian. Continuing the 

 calculation of the track of total eclipse after leaving this island, I 

 find it would enter Palestine near Jaffa, and pass over Jerusalem 

 and Hebron, where the sun would be hidden 4} minutei about 

 3 r. M., and from Nablous on the north to Ascalon on the south 

 the country would be in darkness for nearly the some interval. 

 The magnitude of the eclipse of 11S7, September 4, was rather 

 more than 9-ioths at Jerusalem, the central line passing between 

 eight and nine degrees to the north ; in the eclipse of ngi, June 

 23, ihc magnitude was about 7-ioths. 



" 14. The Eclipse of 1433, June 7, long remembered in Scot- 

 land as "the black hour." — It was a remarkable eclipse, the 

 moon being nearly in perigee and the sun not far from apogee. 

 The central line traversed the country in a south-easterly direc- 

 tion, from Ross to Forfar, passing near Inverness and Dundee. 

 Maclaurin mentions that in his time a manuscript account of 

 this eclipse was preserved in the University of Edinburgh, 

 wherein the darkness is said to have come on about 3 r.M., and 

 to have been very profound. By direction calculation for Edin- 

 burgh I find the total eclipse commenced at 3h. 3m., and con- 

 tinued 3m. 41S. At Inverness totality continued 4m. 32s. The 

 after course of this eclipse was north of Frankfort on the Main 

 and Munich, over the Dardanelles, south of Aleppo, and thence 

 nearly parallel to the course of the Euphrates to the north-east 

 border of Arabia. The totality was observed in the Turkish 

 dominions according to Calvisius. 



" 15. The Eclipse of 159S, February 25. — Maclaurin says the 

 memory of this ecUpse was preserved among the people of Scot- 

 land, and ' that day they termed Black Saturday.' He adds : — 

 ' There is a tradition that some persons in the north lost their way 

 in the time of this eclipse, and perished in the snow ' — a state- 

 ment the probability of which our expedence of recent pheno- 

 mena by no means tends to support. The central eclipse may 

 be described as having passed about five miles south of Stranraer 

 to the Ba^s Rock, a little south of Edinburgh, or, more precisely, 

 over Dalkeith. Totality came on at Edinburgh at loh. 15m., 

 and continued im. 30s. The duration was the same at Douglas, 

 Isle of Man. From the rapid motion of the moon in declination 

 the course of the central line was a quickly-ascending one, in 

 latitude on the earth's surface, the total eclipse passing off within 

 the Arctic circle. Kepler must refer to another eclipse which 

 was observed by Jessenius at Torgau, on the Elbe, though he 

 gives the above date. 



" 16. The Eclipse of 1652, April 8, to which reference is also 

 made by Maclaurin as 'still famous among the populace of 

 Scotland, and known among them by the appellation of Mirk 

 Monday.' — The central line passed over the south-east of Ire- 

 land, near Wexford and Wicklow, arrived on the shores of Scot- 

 land near Burrow Head, Wigtonshire, and running %vithin a few 

 miles from Edinburgh, Montrose, and Aberdeen, left the island 

 at Peterhead. Greenock and Elgin would be situate near the 

 north limit, and the Cheviots and Berwick upon the south limit 

 of totality. The ecUpse was observed at Carrickfergus, Ireland, 

 by Dr. Wyberd. I find by direct calculation for this place that 

 it was only just within the north limit of totality, which would 

 commence at loh. Sm. 30s., and continue 44 seconds. This 

 short duration may partly explain a curious remark of Dr. Wy- 

 berd, that when the sun was reduced to 'a very slender crescent 

 of light, the moon all at once threw herself within the margin of 

 the solar disc with such agility that she seemed to revolve like 

 an upper millstone, affording a pleasant spectacle of rotatory 

 motion.' Wyberd's furtherdescription clearlyapplies to the corona. 



" I beheve it has been generally supposed that the last total 

 eclipse of the sun visible m England was that of 1715, May 3, 

 so well recorded by Ilalley in the 'Philosophical Transactions ' of 

 the Royal Society, and I was under this impression myself until, 

 on calculating the elements of the eclipse of 1724 (May 22), 

 observed at I'aris, and by the French King at the Trianon, I 

 discovered that before reaching France the belt of totality must 

 have traversed the south-west of England, and it now appears 

 that the totality did not pass by us unrecorded. 



"I am indebted to the Astronomer Royal for referring me to 

 an account by Dr. Stukeley, who observed the eclipse from 

 Salisbury Plain. The duration of totality in that locality would 

 be rather less than three minutes. The eclipse of 1724 is there- 

 fore the last that has been t0t.1l in England, and as I have shown 

 in a previous communication, there will be no other till iVugust 1 1, 

 1999, and that will be confined to the sou;h-west corner of the 

 country." 



ON PHO TO GRAPHIC IRRADIA TION IN VER- 

 EXPOSED PLA TES* 



'X'HE most cursory observer of any of the recent corona photo- 

 -^ graphs must have remarked the apjjareiit eating-in of the 

 prominences over the limb of the dark Rloon. A more careful 

 examination of the photographs shows that the whole limb of the 

 Moon is more or less eaten into, and that the indentations under 

 the prominences are only exaggerations of a phenomenon which 

 is present at all parts of the limb, but which varies in intensity 

 according as the dark limb of the Moon is projected on a brighter 

 or less luminous background. 



In all over-exposed photographs of luminous objects upon a 

 dark background, the brighter parts of the picture are found to 

 be surriiunded by a nebulous haze or border of light, which in- 

 creases the diameter of the image formed by the luminous objects 

 at the expense of those which are less luminous. 



This nebulous haze has often been spoken of as " the extension 

 of the chemical action,'' but without begging the question of its 

 cause, we propose tcspeak of it as photographic irradiation. It 

 has been found to vary with the time of exposure, and the rela- 

 tive brightness of the object and its background. 



On examining the effects of photoi-raphic irradiation in a de- 

 cidedly over-exposed picture, it will be seen that the nebulous 

 fringes round luminous objects are distinctly divided into two 

 parts — an inner and very marked border of light, following the 

 contour of the luminous objects, and an outer and much less 

 definite haze, thus:— 



A 



where Fig. A represents a normal photograph, and Fig. B a 

 decidedly over-exposed plate from the same object. 



The inner border of light fades gradually from the inside out- 

 wards, and it is very difficult, and indeed impossible, to tell 

 where the true image of the luminous object ends, and its photo- 

 graphic irradiation begins. While, on the other hand, the boundary 

 between the outer and inner fringes (or halos) of irradiation is 

 more definitely marked, although it would lie difficult to say with 

 any absolute precision, at what point the inner fringe terminates. 



Our first experiments were devised in order to test whether 

 reflections from the back surface of the plate played any part 

 in the production of the fringes ; for this purpose plates of ebonite 

 and the so-called non-actinic yellow glass were prepared. 



In the over-exposed photographs taken on ebonite, it was 

 found that the outer haze had entirely disappeared ; while in the 

 photographs taken on plates of yellow glass the outer haze is 

 still distinctly to be traced, though it is much fainter than on an 

 ordinary white glass plate with the same exposure. 



By placing a piece of wetted lilack paper at the back of an 

 unground plate the outer haze may be greatly reduced, while it 

 was found that by grinding both the back and the front surfaces 

 of a yellow gfass-plate, and covering the back with a coating of 

 black varnish, the outer haze may be rendered quite imperceptible, 

 while, however, the inner border of irradiation still remains as 

 before. 



From these experiments we may conclude that the outer haze 

 is produced by reflections from the back of the plate ; and the 

 action of the wetted black paper in reducing the outer irradiation 

 may be explained by the consideration that the change of refrac- 

 tive index in passing from the glass to the film of water behind, 

 is much less than in passing from glass into air. There is, con- 

 sequently, less reflection at the back surface of the plate ; most 

 * By Lord Lindsay and Mr. A. Cowper Ranyard. Reprinted from the 

 Monthly Notices of the Royal Astronomical Society, June 14, 1872. 



