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282 THE PHOTOGRAPHIC NEWS. [June 13, 1862. part would be slightly out of focus to the same degree— just enough to soften the hardness of the lines ; but this cannot be obtained, because if some parts which would have been in the right focus were softened by a slight alteration in focussing, this would render the other parts already in bad focus perfectly confused and indistinct. The principal object of taking photographic portraits as large as nature is to give to the artist a ground to paint upon, containing a correct representation of all the features, forms, and draperies, which he can follow until the picture is so far advanced that a short sitting of the person is sufficient to impart the exact colour of the eyes, hair, and complexion. But it is evident that as soon as the artist has begun, and the more he advances his work, his colours cover and hide the photograph, and that it becomes very difficult for him to go on without losing the likeness if he had not always before his eyes another copy of the same photograph, to which he can constantly refer, in order to see when he is erring, and which enables him to correct any false touches of his brush. It would really seem that a skilful painter does not strictly want to paint upon the photograph; that he might as well trace the enlarged photograph upon his canvas to form the base of his painting, and to keep the photograph for his guide. In this case he might paint upon a canvas properly prepared to receive the colours, and not affected by the strong shadows produced by the nitrate of silver, upon a surface of paper which has had to undergo a quantity of washings and manipulations. While speaking of the immense difficulty of producing a photograph upon a surface of paper large enough for a portrait of natural size, the chemical substances forming the photograph are capable of affecting the dura bility and constitution of the colours forming the picture. Considering all these difficulties, and while waiting in despair for the sun which was to enable me to produce en larged photographs, it occurred to me that if I could supply the artist with a canvas upon which, while the image of the negative was thrown upon it by the light of the sun, and in its absence by the light of a gas-burner, I had traced every part of the picture with a black pencil, this would form a base oven better than any photographic impression for the painter, who, being supplied with such a perfect lightly- drawn portrait, and, with a photograph for his guide, would have all which is necessary for his work. I have tried the experiment, and it has answered beyond my expectations. In the British Department of Photo graphy at the Industrial Exhibition may be seen several portraits as large as nature, which have been painted on canvas upon which a small negative had been enlarged by pencil-drawings. I have done it myself; and when I add that I am quite incapable of drawing, it must be acknow ledged, by the inspection of the result, that it is entirely due to the help of photography, and is one of its most useful applications. The artists whom I have employed to paint these enlarged photographs have found them prefer able to those produced by the chemical operation upon paper, because they have been able to lay their colours in all their brilliancy, without being impeded by the dark shadows of the ordinary photographs. As an example of the result, I call the attention of the meeting to a portrait of a boy painted in pastel, and to another of Professor Faraday painted in oil—both of which have been completed without sitting. Those to whom the features of the eminent Professor are familiar will be able to judge of the correctness of the likeness, and to appreciate this new mode of applying photography to the production of large portraits. I have thought that this mode of turning the base for large portraits upon canvas, the usual and most; suitable surface for the work of the painter, deserved the attention of photo graphers, who, to satisfy the want of the public, are obliged to employ artists to produce painted portraits. If it is found that this mode of operating is capable of results entirely satisfactory, and that it can be effected at any time when wanted, without the light of the sun, surely this must be considered as a great advantage in one of the most import® applications of photography, which consists in furnishing the artists with a perfect base for painting portraits—hanin all the truth and character of painted photographs, and 8 easy to be obtained. 9 3 Having been the first to practise this process, the idea® which I have no doubt must have occurred to many othert and the productions having been received in the Brits Department of Photography at the International Exhibition I have considered it my duty to explain before this Socie! how these large portraits have been obtained, in order to prevent any misconception as regards their nature. . It is known in optics that when an image is considerabl! enlarged by a lens, the centre only of that image is in conet proportion, because the screen being flat and the imago being curved the parts thrown on the margin of the screen have a focus nearer than the screen, and consequently bein represented on the screen at a farther distance, they are 00 of focus and enlarged. In order to obviate that defed the negative should present, alternately, all its various parb to the centre of the lens, and at every change, horizontally and vertically, a different image should be taken. Havin taken the various parts forming the whole picture, ant cutting them all in order to mount them in one picture," obtain a representation without distortion. When we wsl to draw the picture by hand we have only to bring th various parts of the canvas in the place corresponding wi the movements of the negatives, and we have a petfdd picture of the whole. This plan has another advantage: it enables ustoenW a negative to a degree at which it would be impossible 1 light the whole picture, by the limited centre of light the condenser. The arrangements by which I move t! negative vertically and horizontally will be better undef stood by the inspection of the apparatus and its action. RULE FOR FINDING AT ONCE BOTH THE DIS TANGES OF NEGATIVE AND SENSITIVE SU FACE FOR ANY DEGREE OF ENLARGEMEN- AND VICE VERSA.* BY M. CLAUDET, F.R.S. In a former paper, which was published in the number oft Journal of the Photographic Society for March, 1861,1 b" given a new theory for the measurement of the focal distap. on one side of the lens, and the distance of the object on™ other side. I have explained why both these distances 1 be measured, not from the surface of the lens, but fro 111 point before it, which is the nearest distance at which ’ object may be placed for producing the largest image P sible, and another behind which is exactly the focus, parallel rays giving an image as small as possible. , s space between these points and both sides of the lens 180 nonentity, being in fact nothing more than the thicknedu the partition dividing the two sides of the camera-obsd without lens. aj 'These two points arc the zero of the scales of measurena for the focal distance and distance of object. When tla two points have been determined, if we find two other P° at which the object must be placed before the lens to lb. ducc behind it an image of the same size, these two Pde form the other extremity of the scale, and that scale Sa unity which serves for measuring all the distances ami di8 mining exactly the proportion between the size of the ° J and that of the image. . 1j To facilitate all the calculations these scales are divide one hundred equal parts. . dbe When we want to have an image of the same size 48 - object, we have only to place both the object and these ., on the 100th division of each scale. For an image 1 J te the size of the object we place the object on the halt a scale precisely on the division 50, and the screen ae^z * Read at a meeting of the London Photographic Society, June 3, 19