208 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[June, 



from which, if it l>e found to be decomposed by light, the corrosion of the 

 glass, and ronscqucntly an etching, might possibly be obtained, by the lil]cra- 

 tiori uf tluorinc. As it is known that light reduces the salts of gold and of 

 ]ilatinuur, as well as those of silver, the author was induced to make many 

 experiments on the chlorides of these metals, in reference to the objects of 

 photography ; the details of which experiments are given. A remarkable 

 property of bydriodic salts, applied, nnder certain circumstances, to exalt the 

 deoxidating action of light, and even to call into evidence that actioji, when 

 it did not before exist, or else was masked, is then described. 



4. The chemical analysis of the solar speetrmn forms the subject of the 

 next section of bis pa)icr. It has long been known that rays of different 

 colours and rcfrangiliilitics exert very different degrees of energy in effecting 

 chemical changes ; and that those occupying the \ iolct end of the spectrum 

 possess the greatest deoxidating powers. But the author tinds that these 

 chemical energies are distributeil throughout the whole of the speetrmn ; that 

 they are not a mere function of the refraiigibility, but stand in relation to 

 physical i]ualities of another kind, both of the ray and of the analyzing me- 

 diiun ; and tint this relation is l)y no means the same as the one which de- 

 termines the .absorptive action of the medium on the colorific rays. His 

 ex])eriments .ilso show that there is a third set of rebitions concerneil in this 

 action, and most materially intiuencing both (he amount and the character of 

 the chemical action on each point of the spcctrnm ; namely, those tlepending 

 on the physical qualities oftlie substance on which the rays are received, and 

 whose changes indicate and measiu-e their action. The author endeavoured 

 to detect the existence of inactive spaces in the chemical spectrum, analogous 

 to the dark lines in the luminous one ; but without any marked success. The 

 attempt, however, revealed several curious facts. The maximinn of action on 

 the most ordinary description of jihotographic paper, namely, that prepared 

 with common salt, was foinnl to be, not beyoml the violet, but about the 

 coulines of the blue and green, near the situation of the ray F in Fraunbofer's 

 scale : and tlie visilile termination of the violet rays nearly bisected the pho- 

 tographic image impressed on the paper : in the visible violet rays there oe- 

 cm-red a sort of minimum of action, about one-third of the distance from 

 Fraunbofer's ray II, towards G; the whole of the red, up to about Fraunlio- 

 fer's line C appears to be inactive ; and lastly, the orange-red rays communi- 

 cate to the paper a brick-red tint, passing into green and dark blue. Hence 

 are deduced, first, the absolute necessity of perfect achromaticity in the ob- 

 ject-glass of a photographic camera r and secondly, the possibility of the 

 future ])roduction of naturally coloured photographs. 



5. The extension of the vinible prismatic spectrum beyond the space ordi- 

 narily assigned to it, is stated as one of the results of these researches ; the 

 author having discovered that beyond the extreme violet rays there exist 

 luminous rays affecting the eyes with a sensation, not of violet, or of any 

 other of the recognized prismatic hues, but of a colour which may be called 

 lavoider-yrei/j and exerting a powerfid deoxidating action. 



fi. Chemical properties of the red end of the spectrum. The rays occupy- 

 ing this part of the si)ectnuu were found to exert an action of an opposite 

 nature to that of the bkie, violet, and lavender rays. AVhen the red rays act 

 on ])repared paper in conjiniction with the diffused light of the sky, tlie dis- 

 colourating influence of the latter is suspended, and the paper remains white ; 

 but if the paper has been already discoloured by ordinary liglit, the red rays 

 change its actual colour to a bright red. 



7. The combined action of rays of different degrees of refrangibility is next 

 investigated ; and the author inquires more particularly into the effects of the 

 combined action of a red ray with any other single ray in the spectrum ; 

 whether any, and what differences exist Ijetween the joint, ami the successive 

 action of rays of any two different and definite refrangibilities ; and whether 

 this action be cajiable, or not, of producing effects, which neither of them, 

 acting ahme, would be competent to produce. The result was that, although 

 the previous action of the less refrangible rays does not appear to modify the 

 subsequent effects produced by the more refrangible ; yet the converse of this 

 proposition does not obtain, and tlie simultaneous action of both produces 

 j>hotograpbic effects very different from those which either of them, acting 

 se)iarately are capable of iiroducing. 



8. In the next section, tlie chemical action of tlie solar spectrum is traced 

 much beyond the extreme red rays, and the red rays themselves are shown to 

 exercise, under certain circumstances a blackening or deoxidating power. 



9. The author then enters into a speculation suggested by some indications 

 which seem to have been afforded of an absorptive action in the sun's, atmo- 

 sphere ; of a difference in the chemical agencies of those rays wliich issue 

 from the central parts of his disc, and those which, emanating from its bor- 

 ders, have undergone the absorptive action of a much greater depth of his 

 atmosphere ; and conseipieutly of the existence of an absorptive solar atmo- 

 sjihere extending beyond the luminous one. 



10. An account is next given oftlie effect of the S])ectrum on certain vege- 

 table colours, as determined by a series of experiments, which the author has 

 commenced, but in which the unfavoiu'able state of the weather has, as yet, 

 prevented him from making much progi'ess. 



11. The whitening power of the several rays of 1 he spectnnn under the 

 influence of bydriodic sals, on jiaper variously iirciiared, and previously dark- 

 ened by the action of solar liglit. The singular property belonging to the 

 hydriodate of potash of rendering darkened photographic paper susceptible of 

 being whitened by furtlier exposure to light, is here analyzed, and shown to 

 afford a series of new relations among the diffweut pai'ts of the spectnm)| 

 with respect to theii: chemical actions. 



12. The Analysis of the Chemical Rays of the Spectrum by absorbent me- 

 dia, which forms the subject of the next section, ojiens a singularly wide lield 

 of inquiry ; and tlie author describes a variety of remarkable iibenomena 

 which have presented themselves in the course of his experiments on this 

 subject. Tliey prove that the photographic properties of coloured media do 

 not conform to their colorific character : the laws of their absorptive action 

 as exerted on the chemical, being different and independent of those on tlic 

 luminous rays : instances are given of the absence of any darkening effect in 

 green and other rays of the more refrangible kind, which yet produce con- 

 siderable illumination on the jiaper that receives them. 



13. The exalting and depressing power exercised by certain media, under 

 ]ieciili,'ir circumstances of solar light, on the intensity of its chemical action. 

 This branch of the inquiry was suggested by tlie fact, noticed by the author 

 in his former communication, that the darkening power oftlie solar rays was 

 considerably increased by the interposition of a plate of glass in close contact 

 with the photographic paper. The influence of various other media, super- 

 posed on prepared paper, was ascertained by experiment, and the results are 

 recorded in a tabular form. 



\i. The paper concludes with the description of an Aciinograph, or self- 

 registering photometer for meteorological purposes: its objects being to ob- 

 tain a permanent and self-coin]iarable register and measure, first, of the mo- 

 mentary amount of general illumination in the visible hemisphere, which 

 constitutes day-light; and secondly, of the intensity, duration, .-ind inter- 

 ruption of actual sunshine, or, when the sun is not visible, of that point in 

 tlie clouded sky behind which the sun is situated. In a postscript, dated 

 March 3rd, 1840, the author states that he lias discovered a process by which 

 the colorific rays in the solar spectrum are made to aft'ect a surface properly 

 prepared for that purpose, so as to form what may be called a thennorjraph 

 of the spectrum ; in which the intensity of the thermic ray of any given re- 

 frangiliility is indicated by the degree of whiteness produced on a black 

 ground, by the action of tlie ray at the points where it is recei\ ed at that siu'- 

 face, the most remarkable result of which is the insulation of heat-spots or 

 thermic images of the sun quite apart from the great body of the thermic 

 spectrum. Thus the whole extent over which prismatic dispersion scatters 

 the sun's rays, including the calorific effect of the least, and the chemical 

 agency of the most refrangible, is considerably more than twice as great as 

 llie Newtonian coloured spectrum. In a second note, communicated March 

 12, 1S40, the author describes his process for rendering lisible the thermic 

 spectrum, which consists in smoking one side of very tliin white pajier till it 

 is completely blackened, exjiosing the white surface to the spectrum and 

 washing it over with alcohol. The thermic rays, by dning tlie points on 

 which they impinge more rapidly than the rest of the surface, trace out their 

 extent and the law of their distribution by a whiteness^o induced on the 

 general blackness which the whole siuface acquires by the absorption of the 

 liquid into the pores oftlie paper, lie also explains a method by which the 

 impression thus made, and which is only transient, can be rendered perma- 

 nent. This method of observation is then ajijilied to the further examination 

 of various points connected with the distribution of the thermic rays, the 

 transcalescence of jiarticular media, and the polarization of radiant heat 

 (which is easily rendered sensible by this method), &c. The reality of more 

 or less insulated spots of heat distributed at very nearly equal intervals along 

 the axis of the spectrum (and of which the origin is probalihj to be sought iu 

 the flint glass prism used — but pitssihhj in atmospheric absorption) is estab- 

 lished. Of these spots, two of an oviil form, are situated, the one nearly at, 

 and the other some distance beyond the extreme red end of the spectrum, 

 and are less distinctly insidated ; two, i)erfectly round and well insulated, at 

 gi'eater distances in the same direction ; and one, very feeble and less satis- 

 factorily made out, at no less a distance beyond the extreme red than 422 

 ])arts of a scale in which the whole extent of the Newtonian coloured spec- 

 trum occupies 539. 



A paper was ,also read entitled, " Vemarks on tlie Theory of the Dispersion 

 of l.h/ht, as cnunected with Polarization." By (he llev. Baden Powell, M. A. 

 Since the ]iublicatinn of a former letter on tlie same stdijccf, the author has 

 been led to review the theory in coiiuexion with the valuable illustrations 

 given by Mr. Lubbock of the views of Kresnel; and points out, in the present 

 supplement, in what manner the conclusions in that paper will be affected by 

 these considerations. 



A ]iapcr was also read, entitled, " Further Particulars of the Full of the 

 Cold Bokkeveld Meteorite." By Tliomas Maclear, Es(|., F.U.S., in a letter to 

 Sir J. r. \V. Herscbel, Bart. — This communication, which is supplementary 

 to the one already made to the Society by Mr. Maclear, contains reports, 

 supported by altiilavits, of the circumstances attending the fall of a meteoric 

 mass in a valley near the Cape of (iood Hope. The attention of the witnesses 

 had been excited by a loud explosion which took place in the air, iirevious to 

 the descent of the aerolite, and which was attended by a blue stream of 

 smoke, extending from north to west. Some of the fragments which had 

 been seen to fall, and which had ])eiictrated into the eartli, were picked up 

 by the witnesses. One of them falling on grass caused it to smoke: and was 

 too hot to admit of being touched. The mass which was sent to England by 

 11. M.S. Scout, weighed, when first picked up, four pounds. The pajier is 

 accompanied by a map of the district, showing the course of the aerolite. 



A paper was .ilso read, entitled, " Aa account nf tlic Sliootiny Stars o/'1095 

 and 1243." By Sir Francis Balgrave, K.II.' — The author gives citations from 

 several clironicles of the middle ages, descriptive of the remarkable appear- 

 ance of shooting Stan \ybicU occurred on the -ith of April, 109^, ou the testi« 



