286 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[August, 



plain vaultings of the miilfile ages is, that all the curves are segments of 

 circles, the diagonals being struck from a centre below the springing of the 

 lateral and cross ribs, and are contrasted in this respect with diagonals pro- 

 jected from the direct arches, according to the rale familiar to every carpen- 

 ter, from which it results that all the points of a groined vault coincide, and 

 vrill be touched by a straight line drawn from one end of such a range of 

 vaulting to the other. To this mode of setting out the curves may be attri- 

 buted the flagrant want of character which is apt to distinguish the modern 

 imitations of Gothic vaulting, and it may even be obsened in original ex- 

 amples, that the effect is less pleasing as this coincidence is more nearly ap- 

 proaclied. This is the case with vaultings executed after the four-centered 

 arch came into fashion, in which, although the curves n^ay not be projected, 

 yet there is an approach to greater regularity from the springing of all the 

 ribs being brought to one level. During the Norman period the drawing of 

 the vaults is very rude, and we find it to have been frequently necessary to 

 back up the ext'rados of the ribs in order to bring the spandrils into shape. 

 In the succeeding period of our architecture more care was indispensable, 

 on account of the greater complication of mouldings converging together at 

 the springing, and the free and sketchy manner in which they are managed, 

 and the superfluous mouldings got rid of before they overioad the impost, is 

 much to be admired, and is greatly superior to the method pursued in the 

 I5th centnrv, when the coverging ribs were all brought down to the impost, 

 and died away into a mere bundle of reeds, of which the eft'ect is exceedingly 

 tame and uncharacteristic. Previously to the introduction of the last style 

 of gothic vaulting, fan groinmi], various complicated figures were formed by 

 the introduction of numerous cross ribs, but the mode of construction con- 

 tinued to be the same. The vaulting immediately preceding fan groining, 

 which may, in fact, be considered as a transition style, Professor Willis desig- 

 nated as stellar groining, from the star shapes which usually enter into its 

 composition, and it is remarkable, that in some cases this form is lost in 

 execution, although laid down on the plan, the architect apparently not hav- 

 ing calculated on the effect of perspective, whereas, in others, the artist has 

 evidently depended upon it in order to bring out his design. At length, in 

 fan groining, the compartments become so numerous that the system of 

 separate ribs is abandoned, and the vaults are constructed according to the 

 ancient and modern principle of cut stone. Professor Willis accompanied 

 his lecture by an extensive display of drawings and models, illustrative of 

 the geometrical system upon which he supposed the Gothic architects to 

 have worked in producing these results. 



After the lecture the noble President presented to Mr. Hall the medal of 

 the Institute, which had teen awarded for his essay on iron roofs. 

 July 19. — R. Wallace, Esq., in the Chair. 



Henry Gaily Knight, Esq., was elected an honorary member. 



Mr. Hall's essay on iron roofs, to which the medal of the Institute had 

 been awarded, was read. 



This was the closing meeting of the session. 



CALOTYPE. 



The following account of some recent improvements in photography, by 

 II. F. Talbot, Esq., was lately read before the Royal Society. 



The author had originally intended, in giving an account of his recent 

 experiments in photography, to have entered into numerous details with 

 respect to the phenomena observed ; but finding that to follow out this plan 

 would occupy a considerable time, he has thought that it would be best to 

 put the Society, in the first place, in possession of the principal facts, and by 

 so doing perhaps invite new observers into the field during the present fa- 

 vourable season for making experiments. He has, therefore, confined him- 

 self at present to a description of the improved photographic method, to 

 which he has given the name of Calolype, and reserves for another occasion 

 all remarks on the theory of the process. The following is the method of 

 obtaining the Calotype pictures. 



Prpparalion of the Paper,— Take a sheet of the best writing paper, having 

 a smooth surface and a close and even texture. The watermark, if any, 

 should be cut off, lest it should injure the appearance of the picture. Dis- 

 solve 100 grains of crystallized nitrate of silver in six ounces of distilled 

 water. Wash the paper with this solution, with a soft brush, on one side, 

 and put a mark on that side whereby to know it again. Dry the paper cau- 

 tiously at a distant fire, or else let it dry spontaneously in a dark room. 

 When dry, or nearly so, dip it into a solution of iodide of potassium contain- 

 ing 500 grains of that salt dissolved in one pint of water, and let it stay two 

 or three minutes in this solution. Then dip it into a vessel of water, dry it 

 lightly with blotting-paper, and finish drying it at a fire, which will not in- 

 jure it even if held pretty near ; or else it may be left to dry spontaneously. 

 All this is best done in the evening by candle-light. The paper so far pre- 

 pared the author calls iodized paper, because it has a uniform pale yellow 

 coating of iodide of silver. It is scarcely sensitive to light, but, nevertheless, 

 it ought to be kept in a portfolio or a drawer, until wanted for use. It may 

 be kept for any length of time without spoiling or undergoing any change, 

 if protected from the light. This is the first part of the ]ireparation of Calo- 

 type paper, and may be performed at any time. The remaining pait is best 

 deferred until shortly before the paper is wanted for use. When that time 

 is arrived, take a sheet of the iodized paper and wash it with a liquid pre- 



pared in the following manner: — Dissolve 100 grains of erystalhzed nitrate 

 of silver in two ounces of distilled water ; add to this solution one-sixth of 

 its volume of strong acetic acid. Let this mixture be called A. Make a 

 saturated solution of crystallized gallic acid in cold distilled water. The 

 quantity dissolved is very small. Call this solution B. When a sheet of 

 paper is wanted for use, mix together the liquids A and B in equal volumes, 

 but only mix a small quantity of them at a time, because the mixture does 

 not keep long without spoiling. I shall call this mixture the galh-nitrate of 

 silver. Then take a sheet of iodized i)aper and wash it over with this 

 gallo-nitrate of silver, with a soft brush, taking care to wash it on the side 

 which has been previously marked. This operation should be performed by 

 candle-light. Let the paper rest half a minute, and then dip it into water. 

 Then dry it lightly with blotting-paper, and finally dry it cautiously at a fire, 

 holding it at a considerable distance therefrom. When dry, the paper is fit 

 for use. The author has named the paper thus prepared calotype paper, on 

 account of its great utility in obtaining the pictures of objects with the ca- 

 mera obscura. If this paper he kept in a press it will often retain its quali- 

 ties in perfection for three months or more, being ready for use at any mo- 

 ment ; but this is not uniformly the case, and the author therefore recom- 

 mends that it should he used in a few hours after it has been prepared. If 

 it is used immedialely, the last drying may he dispensed with, and the paper 

 may he used moist. Instead of employing a solution of crystallized gallic 

 acid for the liquid B, the tincture of galls diluted with water may be used, 

 but he does not think the results are altogether so satisfactory'. 



Use of the Paper. — The Calotype pajier is sensitive to light in an extra- 

 ordinary degree, which transcends a hundred times or more that of any kind 

 of photographic paper hitherto described. This may he made manifest by 

 the following experiment : — Take a piece of this pai)er, and having covered 

 half of it, expose the other half to daylight for the space of one second in 

 dark cloudy weather in winter. This brief moment suffices to produce a 

 strong impression upon the paper. But the impression is lateut and invisi- 

 ble, and its existence would not be suspected by any one who was not fore- 

 warned of it by previous experiments. The method of causing the impres- 

 sion to become visible is extremely simple. It consists in washing the paper 

 once more with the gallo-nitrate of silver prepared in the way above de- 

 scribed, and then warming it gently before the fire. In a few seconds the 

 part of the paper upon which the light has acted begins to darken, and 

 finally grows entirely black, while the other part of the paper retains its 

 whiteness. Even a weaker impression than this may be brought out by re- 

 peating the wash of gallo-nitrate of silver, and again warming the paper. 

 On the other hand, a stronger impression does not require the warming of 

 the paper, for a wash of the gallo-nitrate suffices to make it visible, without 

 heat, in the course of a minute or two. A very remarkable proof of the sen- 

 sitiveness of the calotype paper is afforded by the fact stated by the author, 

 that it will take an impression from simple moonlight, not concentrated by 

 a lens. If a leaf Is laid upon a sheet of the paper, an image of it may be 

 obtained in this way in from a quarter to half an hour. Tliis paper being 

 possessed of so high a degree of sensitiveness, is therefore well suited to 

 receive images in the eameia obscura. If the ajicrture of the object-lens is 

 one inch, and the focal length fifteen inches, the author finds that oneminute 

 is amply sufficient in summer to impress a strong image upon the paper of 

 any building upon which the sun is shining. When the aperture amounts to 

 one-third of the focal length, and the object is very white, as a plaster bust, 

 &c., it appears to him that one second is sufficient to olitain a pretty good 

 image of it. The images thus received upon the Calotype paper are for the 

 most part invisible impressions. They may be made visible by the process 

 already related, namely, by washing them with the gallo-nitrate of silver, 

 and then warming the paper. When the paper is quite blank, as is generally 

 the case, it is a highly curious and beautiful phenomenon to see the spon- 

 taneous commencement of the picture, first tracing out the stronger outlines, 

 and then gradually filling up all the numerous and complicated details. The 

 artist should watch the picture as it developes itself, and when in his judg- 

 ment it has attained the greatest degree of strength and clearness, he should 

 stop further progress by washing it with the fixing liquid. 



The friny process. — To fix the picture, it should be first washed with 

 water, then lightly dried with blotting paper, and then washed with a solu- 

 tion of bromide of potassium, containing 100 grains of that salt dissolved in 

 eight or ten ounces of water, .\ftcr a minute or two it should be again 

 dipped in water and then finally dried. The picture is in this manner very 

 strongly fixed, and with this great advantage, that it remains transparent, 

 and that, therefore, there is no difficulty in obtaining a copy from it. The 

 calotype picture is a negative one, in which the lights of nature are repre- 

 sented by shades ; but the copies are positive, having the hghts conformable 

 to nature. They also represent the objects in their natural position with 

 respect to right and left. The copies may be made upon Calotype paper in 

 a very short time, the invisible impressions being brought out in the way 

 already described. But the author prefers to make the copies upon photo- 

 graphic paper prepared in the way w hich he originally described in a memoir 

 read to the Royal Society in February 1839, and which is made by washing 

 the best writing paper, first with a weak solution of common salt, and next 

 with a solution of nitrate of silver. Although it takes a much longer time 

 to obtain a copy upon this paper, yet, when obtained, the tints appear more 

 harmonious and jdeasing to the eye ; it requires in general from three mi- 

 nutes to thirty njinutes of sunshine, according to circumstances, to obtain a 

 good copy on this sort of photographic paper. The copy should be washed 



