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July 27, 1883.] THE PHOTOGRAPHIC NEWS. 475 with a lens whose focal distance is known (say f inches) : at what distance from the object must the optical centre of the lens be placed, and at what distance the focussing screen ? The rule is, to the focal length of the lens add the focal length divided by the magnification (or diminu tion) required, and this will give the distance of optical centre of the lens from the screen. Multiply this distance by the magnification (or diminution), and that will give the distance of the object from the optical centre of the lens. [In mathematical formula it is expressed— v=/+, where v is distance of the optical centre of the lens from the object. This formula is arrived at by the formula for conjugate focal distances. where 1.=1 4- — f u v f being the equivalent focus, v and u the distances of the object and the screen from the lens. Since the enlargement depends on the proportion of v to u, if n be the number of times enlargement 1=1+1 f nv • o=/+. u=nv=n(f+1 " J Example.—Suppose we want to enlarge a negative four times with a lens of 10-inch equivalent focus: where is the object to be placed? 10+10=124 inches. inches respectively. What would be the equivalent focal length ? F= 12X10x8 = 960 = 3,8, or 31 in. nearly. 12x104-8x104-12x8 296 J In ordinary photographic doublets, the lenses are separated by an interval, in which case the rule to apply is Multiply the focal length of one lens by that of the other, and divide by some of their focal lengths, less their distance apart. Thus, in the case of a symmetrical doublet of 16-in. focus for each lens, and separated by one inch, the equivalent focus would be— 16X16 -8A in., or 8J nearly. If a concave (or any lens which was thinnest in the centre) had to be combined with a convex lens (or any lens which was thickest in the centre), the same rules would apply, only in that case the principal virtual focal length of the concave would have to be substracted from the principal focal length of the convex lens. The formula would be, when /is focal length of the convex lens, and f‘ the virtual focal length of the concave lens— 1 _ 1 _1 F f / F=ff x or ~f l -f The above will be found useful to photographers who wish to use a lens with longer focus, when they only have a doublet to use. It will be seen that by unscrewing one lens, and using only one of the combination, a much longer focal length can be obtained. To find the distance of the lens from the screen, we have to multiply the distance lines obtained by 4, which, in this example, would be 50 inches. Allusion has already been made to a combination of lenses. It may be well here to point out what equivalent focal length we shall obtain by combining two lenses together. Suppose we have two lenses having focal lengths / and f, respectively; then, if the lenses were very thin and placed close together, the equivalent focal length of two lenses is found by multiplying together the principal focal lengths of the two lenses, and dividing by their sum. [Putting it in mathematical formula)— 1—141 orF=/1] Example.—Two similar lenses, each having an equivalent focus of 16 inches : what would be the effect of combining them together ? This would give us— F_16-+16—8 inches. 32 Again, take one lens of 16 inches, and another of 10 inches; in this case— F=16+10=6, inches. Again, if we had three lenses to combine together, the equivalent focus would be found by multiplying them together, and dividing by the sum of every two and two multiplied together. [The formula would be thus, where// f2 were the three focal lengths:— 1—111 2 ” ff fu orp_ ff+ffu+ffi 7 ffifn - Example.^Three lenses have focal lengths 12,10, and 8 DRY PLATES VERSUS WET. BY E. E. CADETT. A great deal is being said about the retrograde movement photography has taken since the introduction of the gela tine dry plate. In fact, it is said that those photographers who were making fine pictures with the wet process) and who have adopted dry plates, are now getting inferior results. Of portraits on dry plates, there is not much advance, excepting where instantaneous photography comes in ; but this is in a great measure the fault of dry plate manufacturers, and could be easily remedied. When anew batch of plates is bought, one has not the remotest idea of the sensitiveness of the plates, or wbat exposure to give ; but this might be easily overcome by testing a plate out of the batch for yourself, if it were not that some makers have contracted the bad habit of mixing the plates of different batches ; therefore, in such cases it is impossible to tell whether two plates are alike in sensitiveness, as the great difference which may occur between any two differ ent batches is only too well known by any one who has experimented in gelatine emulsion making. Now, if those plate makers who do mix their batches of plates would only keep them separate, they would confer a great favour on the consumers, as it is the cause of a great many failures. Another thing I might mention is putting some test number on the plate, such as the highest number the plate will show when tested by Warnerke’s sensitometer, or any other standard of sensitiveness, so as to give the consumer some idea of the right exposure. This would remove a great drawback from the path of dry plates. This is one of the advantages the wet process possesses over the dry process. Once the bath is in good working order, there is very little difference in the sensitiveness of the plates, pro vided the same sample of collodion is used. Another reason that better results have been got by the wet process is, that the plate must be developed on the spot, and if the results are not good, another plate is taken, and a good picture secured; whereas, with the dry plate process, the exposed plate is generally developed on