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THE]PHOTOGRAPHIC NEWS. stock bottle, and dilute it with one gallon of water, which will be sufficient to tone eight or ten sheets of fumed paper. Toning.—Pour the toning solution out into a clean shallow dish, the larger the better. Porcelain dishes of various sizes are sold for the purpose, and should not be used for any other. Place about twenty or thirty prints in the liquid, one by one, face downwards, agitating the while by rocking the dish, which must be so placed that Very subdued white light may fall on it when required, so that the colours may be properly judged. After the space of five minutes, the prints should be turned face upwards, one by one, when it will be seen that they are still red. If the dish is large enough to allow of it, another twenty prints may be put in as before, face downwards; a little white light is now allowed to reach the prints. Commence by turning over half-a-dozen of the prints last added, laying them face upwards in one corner of the dish, against the source of light, but in the solution ; next pass each print of the first twenty or so towards the half-dozen red ones. A slight difference in colour will be noticed, but not suf ficient to warrant the removal of any one print; draw them back again carefully through the solution, and pass them again, one by one, to the same corner of the dish, this time turning up the remainder of second addition. Some of the prints first added, when compared with those freshly turned up, will be found to be verging in colour to a warm violet. If warm tones are desired, the prints of that shade should be removed from the bath, and placed in a vessel of water containing one ounce per gallon of common salt, which arrests further action of the toning ; ordinary water does not do so perfectly. After a few prints have been removed from the toning solution, more may be added from the washing water, face downwards, and treated precisely as the others have been, until the whole batch are toned. If cold tones are required, the prints should remain for a longer period in the toning bath. When they arrive at the blue stage they may be removed. If any doubt exists in the student’s mind as to whether the print be sufficiently toned or not, he will find it a good plan to hold it up against the source of light ; the colour, as seen by transmitted light, will be pretty nearly the colour ot the print when finished ; but notwithstanding all that has been said, practice is the best guide for indicating the exact time to remove the prints, some papers becoming much warmer than others during the after process of fixation. A few points essential to good work in ordinary practice should be impressed on the student as follows:—Always cleanse your hands and the vessels to be used previous to soaking the prints in water. When the toning is finished, pour the solution back into a clean jar, to be kept only for the purpose, and place it on the shelf protected from dust and light. When again required, pour off the clear solution, and throw the sediment found at the bottom into the residues; add a measured quantity of stock solution, and stir it up well before the prints are added—i.e., one ounce or less per sheet. Avoid placing too many prints in the bath at one time, and keep them constantly agitated. The number of prints above recommended applies only to small sizes. When toning 10 by 8 or 12 by 10 prints, not more than two or three should be in the bath at one time, unless a larger quantity of solution is employed than we have mentioned. Do not use the solution stronger than indi cated in the formulae. Twenty to thirty minutes will be about the average time required when a good sample of terchloride of gold is used. After the first washing and toning vessels have been washed and put in their places, give the prints two changes of water to free them from traces of the toning solution. They are now ready for the next operation, “ fixing,” which will be described in the next lesson. Note.—Subdued white light admitted in the printing room during toning must be used with extreme caution, as the prints are not safe until after fixation. NOTES ON PHOTOGRAPHY. BY E. HOWARD FARMER. Lecture XVII.—Optics* The Refraction of Light and Lenses.—When a ray of light passes obliquely from one transparent medium to another of a different character or density, it is refracted (bent out) from its former direction. If a perpendicular is drawn on the surface separating the two media at the point where the light enters, it is found that when the second medium is denser than the first, the light is refracted towards this perpendicular, and when it is rarer than the first, away from the perpendicular. In passing through an ordinary sheet of glass, light is bent in this way ; but, owing to the second surface being parallel to the first, the second refraction is equal to the first, and, from the foregoing rule, in the opposite direction, so that the emergent light proceeds in a path parallel to its former one. When the two surfaces are not parallel with each other, the emergent light makes an angle with its former path, which is greater, the greater the angle enclosed by the surfaces. On examining these cases also by the rule given, it will be found that the light on emergence is always refracted towards the thickest part of the glass. Depending on these facts lenses are made ; when thickest in the centre they cause light passing through them to converge towards the centre, and when thickest at the margin to diverge towards the margin. The first kind are called converging lenses, and the second kind diverging lenses. There are three forms of each— 1. Double convex 2. Plano convex (one side flat) 3. Convexo-concave (convexity greatest) 4. Double concave 5. Plano concave (one side flat) 6. Concavo-convex (concavity greatest) Nos. 3 and 6 are usually called meniscus lenses, either converging or diverging. Whenever light passes through a lens, the emergent ray is weaker than the incident one, more or less being invariably lost by reflection from the two surfaces. Chromatic Aberration (dispersion).—White light, after passing through a single lens, is found to be fringed with colour, and objects viewed through it also appear fringed with colours; this is due to dispersion, or that the differ ent coloured lights are refracted by the lens to different extents. It is corrected in converging lenses by combin ing with a converging crown glass lens a diverging flint glass lens; the dispersion of the two lenses being oppo site in direction, neutralize one another; the dispersion with flint glass being greater than with crown glass for an equal amount of refraction, enables the required refraction to be obtained. Such a lens is said to be achromatic spherical aberration. Owing to the spherical curves given to lenses, rays from the same point falling on their margin are not brought to the same focus as those falling on their centre, the consequence being that images formed by them are blurred and indistinct. This evil is reduced to a minimum by the employment of small sections of curves, and by the use of stops. Curvature of Field.—Sometimes, on focussing the centre of an object on the ground glass, the margins are found to be indistinct, and if the margins are brought into focus, the centre becomes indistinct. This is due to the fact that the natural focus of an image formed by a lens is curved, not flat. In order to obtain a sharp image on a flat sur face, a meniscus form of lens with the concave side next the object, is employed, and a stop used some distance in front of it. In this case the centre of the lens is used to form the centre of the image, and the margins of the lens nearest the respective margins of the plate to form the • Bead “A Treatise on Photography ” (chap, xxix.) converging lenses. diverging lenses.