July 26, 1894] 



NATURE 



307 



iiructed that the reaction within the tube can be observed, in 

 order to be able to attain any desired temperature. When the 

 ferric oxide is healed to about 600' in the stream of ammonium 

 chloride vapour small glittering crj'stals commence to form after 

 the expiration of a few minutes, the remainder of the oxide 

 ncreases considerably in volume, and ammonium chloride is 

 rapidly absorbed. No fusion occurs, so that the absorption is 

 a mechanical one ; the ammonium chloride condenses upon the 

 exterior of the particles, eventually converting the powder into 

 a while mass. Upon subsequent microscopic examination of 

 ' this white product larjje numbers of the small brilliant crystals 

 I of haematite are observed interspersed among the whitened 

 I particles. When the experiment is performed at 700°, the 

 I whole of the ferric oxide is converted into miniature 

 cryslals of hjematite ; it is piobable that the ammonium 

 chloride is totally dissociated at this tern perature. The crystals 

 exhibit all the pec uliar crystallographic properties of ha;matite. 

 The fundamental rhomlj^hedron possesses the characteristic 

 angle of 86', and the subsidiary forms developed, including 

 those of the sc.ilenohedron, are precisely those exhibited by 

 the natural mineral and are developed to about the same extent. 

 This mode of synthesising hxinatite is very probably intimately 

 connected with that described by M. Sainle Claire Deville in 

 l86i. The latter method consisted in heating ferric oxide to 

 redness in an indifferent atmosphere into which traces of hydro- 

 chloric acid gas were admitted. As the ammonium chloride in 

 the experiment at 700° was most probably completely disso- 

 ciated into hydrochloric acid and ammonia, it is extremely 

 likely that the crystalli-ing action was due to the free hydro- 

 chloric acid. The synthesis of hematite by means of partially 

 "I totally dissociated ammonium chloride vapour is interesting, 

 liuwever, as throwing light upon the mode of formation in 

 nature, for the vapours evolved by the fumaroles in volcanic 

 districts always contain a certain proportion of sal-ammoniac, 

 and it is usually observed that the fissures through which 

 these vapours pass are more or less covered with crystals of 

 -jjecular iron and hremaiite. There is every probability, there- 

 fore, that the formation of the crystals is due to the partially 

 dissociated sal-ammoniac, just as in the artificial experiments 

 above described. 



The additions to the Zoological Society's Gardens during 

 (he past week include a Moustache Monkey {Ccrcopitlucus 

 cephits) from West Africa, presented by .Mr. Clayton Pickers- 

 gill ; a Leopard '^FlUs pardiis) from East Africa, presented by 

 .Mrs. J. R. W. Pigott ; a Lioness {Felis Uo) from East Africa, 

 presented by Major Oiven ; two Tiger Cubs [Fdis ti«ris) from 

 Pehang, Malay Peninsula, presented by Lieut. -Colonel Sir 

 Charles B. H. Mitchell, K.C.M.G. ; a Common Jackal (Ca«»:r 

 aurtui) from India, presented by .Mr. Gerard Garney ; a Monk 

 Seal (Monachus albivenlcr) from Madeira, presented by Mr. C. 

 F. R. Blandy ; a Cockateel [Ca!i'/>silla novizholtamlia:) from 

 Australia, presented by Miss Sloane Stanley ; six South 

 African Erancolins (Francoliniis a/cr), a Puff Adder ( Vipcra 

 arieiaiis) from South .\frica, presented by Mr. J. E. Matcham ; 

 a Smooth Snake (Corom-lla lavii] from Hampshire, presented 

 by Mr. Willingham F Rawnsley ; a Long-eared Fox {O/ocyon 

 megatons) from Somaliland, a Geoffroy's Terrapin {FlaUmys 

 gtoffroyana) from the Argentine Republic, a Ceylonese Terra- 

 pin (Clemmys Irijidga) from Ceylon, an Ocellated Monitor 

 [yaranus ocellatiis) from Lake Tanganyika, two Black and 

 White Snakes {Pitnophis tnelanolcucus) from New Jersey, 

 U.S.A., a Black-winged Peafowl {Pavo nigripennis) from 

 Cochin China, deposited ; a Muscat Gazelle {Gazelta miisca- 

 Uttsii) from Muscat, received in exchange ; two Collared Fruit 

 Bats (Cynoiiycterii eollaris), four Mandarin Ducks {/Ex galcri- 

 cu/ala), six Australian Wild Ducks (Anas superciliosa), two 

 NO. 1291, VOL. 50] 



Slender Ducks {Anas gibberifrons), a Magellanic Goose 

 (Bernicla magellanica), a Black-headed Gull {Larus riJi- 

 btiiidtis), bred in the Gardens. 



OUR ASTRONOMICAL COLUMN. 



The Results of Lmprudent Solar Observations. — 

 Dr. George Mackay, of the Royal Infirmary, Edinburgh, has 

 sent us a pamphlet "On Blinding of the Retina by Direct 

 Sunlight" (J. and A. Churchill), being a study in prognosis, 

 based chiefly upon accidents incurred during the observation 

 of partial solar eclipses. 'I'yros in observations of the sun, 

 and also many incautious astronomers, have sustained more or 

 less permanent injury to the sight by looking at it or its image 

 without the interposition of a dark glass, or similar absorber, 

 of sufficient thickness. During the progress of partial solar 

 eclipses, the laity often make incautious observations, and the 

 results of gratifying such curiosity have furnished Dr. Mackay 

 with the chief part of the clinical material for his study. The 

 paper, which originally appeared in the Ophthalmic Review, 

 opens with a historical survey of the few cases of ocular injury 

 from exposure to sunlight, recorded in historical literature. 

 There is a tradition that Galileo seriously impaired the sight of 

 his right eye by his solar observations, but Dr. Mackay has not 

 been able to trace the story to its origin. It is well known that, 

 in his later years, Galileo became quite blind, but the loss of sight 

 was apparently caused by an affection of the cornea, and not 

 by injury to the retina. The earliest precise description of the 

 subjective sensations consequent upon focussing solar rays upon 

 the retina is due lo Reid, a Professor of Moral Philosophy in 

 the University of Glasgow. He observed the transit of Venus 

 in May 1761, without taking any precautions to modify the 

 intensity of sunlight, and the result was that he was afflicted 

 with metamorphosia ; that is, objects appeared to him in dis- 

 torted forms. Very few other cases of similar ocular injury 

 have been described. Fortunately for Dr. Mackay, the partial 

 eclipse of the sun in June 1890, and that of June 1891, both 

 visible at Edinburgh, furnished him with seven new cases of 

 " Eclipse Blinding," all of which he examined with great care, 

 both with the ophthalmoscope and with type and colour- 

 tests. The patients suffered from an impairment of visual 

 acuteness, and, to most of them, dark spots appeared in their 

 fields of vision. Sometimes these spots were fixed, and in 

 other cases they oscillated rapidly. Dr. Mackay says that com- 

 plete recovery from the injury, even in cases of only slight 

 failure for test-type, is exceptional if investigated by sufficiently 

 refined methods. It is pointed out that the treatment ought to 

 be preventive ; smoked and coloured glasses of the feeble 

 shades ordinarily used by the public to view solar phenomena 

 are quite insufficient. Experience shows that, lo view the 

 sun with impunity, even in January, it is necessary to use a glass 

 so dark that no object illuminated by diffuse daylight is visible 

 through it. 



A Novel Method of Solar Observation. — Dr. Des- 

 landres made an important communication to the Paris Academy 

 of Sciences on July 9. In December 1893, he suggested that 

 separate photographs of the sun should be taken by means of 

 the light of individual dark and bright lines in the solar spectrum. 

 The success with which Prof. Hale has done this with the light 

 of the Iv line shows that striking results may be expected from 

 the development of the method. An ordinary photograph of 

 the sun is mainly produced by the action, upon the sensitive 

 plate, of the bright intervals between dark lines. Dr. Janssen's 

 marvellous pictures of the sun are produced by using only light 

 of high aclinic power, and covering but a small region of the 

 spectrum, to act upon his photographic plate. By carrying this 

 principle still further, there can be no doubt that solar physics 

 will be considerably advanced. The dark lines in the solar 

 spectrum are only dark by contrast. Both Prof Hale and Dr. 

 Deslandres have shown that sun pictures can be produced by 

 the light from them alone. I lence, by isolating aline due to any 

 element, and using it to act upon a sensitive plate, a photograph is 

 obtained of the layer of the sun in which that paiticular element 

 predominates. Dr. Deslandres exhibited to the Paris .\cademy 

 some of the photographs obtained in this way. His first results 

 were produced by means of the light from the bright interval 

 between two dark lines. The pictures thus obtained showed 

 the photosphere with spots and facul.-v much the same as 



