206 



NATURE 



{July 1 6, 11874 



ments were mainly willi dry plates, I will leave out of question 

 llie forms which the phenomenon may assume in wet-plate 

 photography, and summarise the results of hundreds of experi- 

 ments witli dry plates iodised, bromo-iodised, and bromised. 



Witli a simply bromised film the amount of irradiation is 

 extreme. The film is very translucent and the irradiation is of 

 two kinds, that caused by reflection from the back of the plate 

 being by far the most extensive, but remediable by the usual 

 expedient of coating the back of the plate with red or black 

 colour, while the form noticed by Mr. Aitken is perhaps partially 

 inherent in bromised films, but to a much greater degree depend- 

 ent on the nature of the pyroxyHne. Two samples of pyroxyline 

 made at different temperatures, and treated in precisely the same 

 manner, differ somuch,,that while one will, with the coloured back- 

 ing, give scarcely a perceptible degree of irradiation, tlie other will 

 develop it to an extent which no backing, nor even tinting the film 

 with the aniline i-ed?, will obviate. Theformer is generally a com- 

 pact, lustrous film, scarcely to be d stinguisUed from the glass 

 Itself, while the other (Ijoth being used ^^•ithout preservative 

 solution) will give a dull and dusty-looking surface, only capable 

 of reflecting at very small angles. If with the latter a strip of 

 blackened wood be laid on the film so as lo cut across the lightest 

 portions of the image thrown on i. by tlie lens, the effect of the 

 light will hi found to spread 1 ehind the strip of wood, some- 

 times to the extent of a centimetre ; but 1 have never noticed the 

 sharp limitation of this form of irradiation which Mr. Aitken 

 observes, and which probably depends on the wet state of the 

 film. It is clearly, as he supposes, an agi'ation which is set up 

 in the film, and which depends for its propagation amongst the 

 surrounding molecules upon a kind of chemical transparency in 

 the film holding the bromide of silver. That this is to a great 

 extent true is shown by two experiments: (i) a film which, in its 

 simple state, gives eonsidenable halation, will, when coated 

 with albumen, especially if coagulated with nitrate of sil- 

 ver, give none at all, or very little, though the ocular trans- 

 parency is rather increased than diminished by the albumen ; 

 (2) an emulsion prepared by exposing [it to the action of 

 nitrate of silver until it becomes structurally decomposed, and 

 highly charged with bromide of silver, shows absolutely no irra- 

 diation under any circumstances even if the glass be not backed, 

 and no kind of preservative used. The film in this case resembles 

 unbaked porcelain in its whiteness, entire want of lustre, and in 

 opacity, and the molecules of bromide of silver are more than 

 usually free from any restraining influence which a preservative 

 might be expected, reasoning from the usual action of the albu- 

 men, to exert. In these two cases of extreme translucency and 

 opacity of the film there is almost an equal freedom from the 

 phenomenon in question. 



In the old albumen process with translucent films the irradia- 

 tion is imperceptible, and in the coUodio-albumen, where the 

 film of albumen is allowed to remain on the collodion, it is 

 almost so ; but in this case, as in all cases where the film is 

 charged only with ioJide of silver, there is another element 

 which complicates the action. The bromide of .'-ilver is reduced 

 in situ while the iodide requires a supply of silver from the de- 

 veloper from which to build up the image, in the one case the 

 deposition being by reduction, in the other by accamulation. 

 This alone would account for a wide difl'erence in respect to 

 irradiation, but will not account for all, as is proven by the 

 diverse results obtained from different bromide films, due to the 

 varying structure of the material which holds the bromide in 

 place. 



What Mr. Aitken calls "molecular irradiation " (and which 

 is not by any mean; the harmless thing he considers it in regard 

 to artistic photogr.jphy any more than to scientific) is unques- 

 tionably the great enemy of all photogr.iphic precision. It 

 se-m?, how ever, to be complicated with what I have been obliged 

 to call structural irradiation, alluded to above, and depending, as 

 I have said, on the mechanical raither than the chemicd con- 

 dition of the pyroxyline of which the bulk of the film consists. 

 The subject yet demands much investigation, of a purely ein- 

 piric.al cliaracter, in order to determine the quality of vehicle for 

 carrying the sensitive salts, neither chemical analysis nor chemical 

 analojy affording any indication of the true cause of the differ- 

 ence between the two qiiali;ies of pyroxylene 1 have noted, nor 

 do they, so far as I am aware, account for the difference between 

 the action of collodion and albumen. 



\V. J. S 111,1, MAN 



Altenburgh Gardens, 



Clapham Common, S.W., July 13 



OBSERVATORIES IN Till' UNITED STATES''' 



II. 

 T lEUT. M. F. MAURY was placed in charge of 

 -L-' the new U.S. Naval Observatorj-, and entered 

 on his duties with zealous purposes. He proposed in 

 1846 the immense astronomical work of a more ex- 

 tensive and precise cataloguing of the stars than Bes- 

 sel's " Zone Observations " or Struve's " Dorpat Cata- 

 logue." Valuable results of the scheme, so far as it could 

 be entered on, by the observations of Profs. Coffin, Walker, 

 Yarnall, Hubbard, Keith, Major, and Ferguson, and 

 Lieutenants Almy, Maynard, Muse, and others, have been 

 lately reduced and published. 



Two events marked this early part of the history with 

 still more importance. Walker, in 1846, proved that the 

 new planet Neptune, just then discovered by Leverrier, 

 had been catalogued as a star by Lalande in his " Histoire 

 celeste" in 1793 : and Walker, with Lieutenants Almy 

 and Gilliss, was the very first to use, in 1 846, the new dis- 

 covery of the telegraph to determine differences of longi- 

 tude. The identification of Neptune with Lalande's star 

 gave astronomers, in determining the new planet's orbit, 

 the use of observations made fifty-two 5 ears before. It 

 gave the ^;«t7v'(V?« Nautical Almanac two years earlier 

 ephemerides for the mariner. It brought the observatory 

 into prominence. The superiritendency of Maury e.K- 

 tended from 1S45 to April 26, 1S61, when he suddenly left 

 the city to join the cause of the South. 



In 1 861 Lieut. J. M. Gilliss was at length placed in 

 charge. He re-established and vigorously pressed for- 

 ward astronomical work as well as the duties of the " Hy- 

 drographical Office," a title which had been added to that 

 of the Naval Observatory. After his very sudden death, 

 his successor, Rear-Adm'ral C. H. Davis, carried for- 

 ward the astronomical work with that eminent success 

 which had been guaranteed by his previous astronomical 

 tastes and occupancy on the Coast Survey and as super- 

 intendent of the Nautical Aliiianac. Rear-.'\dmiral li. F. 

 Sands, succeeding him in the year 1867, has most efli- 

 ciently improved the opportunities of a longer suptrin- 

 tendency to inaugtiratc and carry forv.'ard some of tie 

 most important astronomical operations of the day. The 

 phenomena of the total eclipses of 1869 in the United 

 States and of 1870 in the Mediterranean countries were 

 closely observed. 



Bc}ond the regular and severely exacting astronomical 

 routine of observations, two centres of interest have been 

 recently occupying the utmost activities of the institu- 

 tion ; the reception, mounting, and use of the new great 

 equatorial, and preparations for the transit of Venus. 



The great equatorial has but one near approach to it- 

 self in the diameter of its object-glass — that of the pri- 

 vate establishment of Mr. R. S. Ncvvall, at Gateshcad- 

 on-Tyne, whose telescope has an object-glass of 25 in. in 

 diameter. The Naval Observatory glass has 26 in. clear 

 aperture. It is not easy to realise what this power is, arid 

 what it promises. The reader must imagine himself 

 within a dome, itself 41 ft. in diameter and 40 ft. in height, 

 looking through a tube tnade of three sections of steel 

 stretching away for 32 ft. ; the whole telescops and its 

 metallic base weighing about 6 tons. 



In the dome, on a pier of mason- work, supported by a 

 pedestal, which is one block weighing 7i tons, stands the 

 tine equatorial made by Merz and Mahler, Munich, at a 

 cost of 6,000 dols., its object-glass being valued at more 

 than half that sum. The work of this instrument under, 

 successively, Profs. Ferguson, W.iiker, Hubbard, and Hall, 

 has been chiefly upon the smaller planets, tlic asteroids, 

 and comets. Mr. James Ferguson was the liist American 

 t3 discover an asteroid, Euphrosyne, in 1854, the thirty- 

 first on a list which has been recently enlarged beyond 

 even a hundred by Peters of Clinton and Watson of Ann 



* continued from p iSj, 



