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862. THE PHOTOGRAPHIC NEWS Vol. VI. No. 188.—April 11, 1862. cd, stop' ociety on value of ject, and • advice; f they do re not in e work of purpose, reful and ■ manipu- •able that ice to the • surface, feet con- •ut silver, return to nd apply eption of imen are pose; the : wc hare cannot, of in entire i pays its ne ? ’ a double e such an the result ompetent k for Mr. not think uses : the Iver bath, ire it and ittle more keep thi 1 to clear i this and largely as '-albumen nt autho- e. Where dry plates ed ands M'ul mani” pillar flow reviously osulphate bly damp Id aid the ; i miner* 5 ill answef lied. 0n f Common e to kee? luted with s much nJ ve. No. * and over t it shoul" •ed by e" nailer th, renter the same tin the circle course th’ f the PlS d picture dry plat nd carets .3. lass room- o was to ion whic posulphi" e of eft in lo” 1, . "pa er, and 6 00, or fi n requin roduce strength: You wh . rd voln!e PHOTOGRAPHY IN AMERICA. We have much pleasure in announcing that we shall in future be able to keep our readers very definitely informed of the progress of photography in America. We have derived, from time to time, much information which has been interesting and instructive from the pages of the American journals and from various transatlantic cor respondents. We hope in future to supply that information in a more systematic form by direct correspondence with F. F. Thompson, Esq., Secretary of the American Photo graphic Society, and principal of a banking-house in New York. Wc append his introductory note, from which it will be seen that amateur photography is rapidly on the ascendant in the States, and that the dry collodion processes, which, until recently, received but little attention, arc re ceiving considerable attention with very successful results:— “ 2, Wall Street, New Yorii, March, 20. 1862. “ DEAR Sib,—Being one of your American subscribers, and having benefited much by reading your News, 1 think it would be a small return for me to send you whatever I can which may be of use to you. “ The number of amateurs in our country is rapidly increasing, and in time we expect to be as numerous as you are in the motherland, and even more so, for the American temper grasps fashion quickly, and a fashion here develops into “the rage.” Wc have already organized “The Amateur Photographic Exchange Club,” an informal association of plan we are describing simplifies the matter; now remove the lens altogether, and measure the distance, from the object focussed, to the ground-glass ; one-fourth of that distance will be the equivalent focus of the combination of lenses, no matter what their construction. To illustrate : we have just tried this method upon a Dallmeyer’s No. 1 triple achromatic lens in our possession, No. 2282. Extending the camera and focussing an en graving, carefully adjusting the length of the camera until the engraving and its image were exactly the same size, and the image perfectly sharp, we then removed the lens and measured the total distance between the engraving and the ground-glass: this we found to be 31,% inches. Dividing this by 4 we obtain 72 inches, ‘which is the equivalent focus of the lens in question. We may incidentally observe that measuring from the central stop to the ground-glass, we also found that in this instance the distance was as nearly as possible the same as the equivalent focus. This plan is so simple in practice, and so correct in result, that our readers will have no trouble in future in ascertain ing the exact focal length of any lens in their possession. Regarding another interesting question associated with this, the method of ascertaining the extent of angle included, by the relation between the area of definition and the equi valent focus, it is not so easy to state the matter in a simple and popular form for the unscientific reader. But as it has become of late a favourite practice of the advocates of various lenses, to state that they include an angle of 60°, that state ment may be very easily tested. If the diameter of a circle of perfect or satisfactory definition be equal to the equiva lent focus of the lens, then it includes 60° in a circular pic ture ; but as most views are cut into the form of a parallelo gram, the extent of definition on the horizontal line should equal the equivalent focus of the lens, in order to include an angle of GO 0 as popularly understood. TO ASCERTAIN THE EQUIVALENT FOCUS OF A COMBINATION OF LENSES. T5 question is frequently asked: What is the simplest and best method of ascertaining the equivalent focus of a combination o f lenses ? Photographers arc rarely familiar with tie exact equivalent foci of the various lenses with h they are working. It has happened that amongst opticians the custom has obtained—more to meet the iormed be three inches in height; suppose also the height tho image of the compound lens under the same circum- stane - be two inches; then as three are to twelve so are two to eight : eight inches then would be the required focus of the combination. Now, although this is a very simple, and , at first sight appears a very correct mode of going about the busners; yet it is troublesome, and involves some sources I error ; the standard of comparison—the focus of the spec- la eye—being an uncertain thing. Another method, -ich renders necessary a series of processes with a telescope -which, by the way, not every photographer possesses— atill more troublesome and complicated, and involves, terefore, still more sources of error. To avoid all this ■•rouble, it has been sometimes recommended to get an approximate idea of the equivalent focus by computing it nu the central stop, or from a point midway between the frol I and back lens. This method involves too much of sumption, and is altogether too much of the character of guess work, to be of any scientific value. Happily there is, however, a much simpler method than “!y of these, and more free than any of them from sources i of error. Let the querist proceed as follows :—take a map, an engraving, or sheet of letter-press, and fasten it flat Aga nab a well-lighted wall; now mark distinctly oft a por- tion say three inches. Proceed to draw out the camera and 9 the subject accurately, taking care that the image is XAct size of the original, which will be ascertained a by observing that the three inches marked off in the o measure exactly those dimensions on the ground- ..As most of our readers know, the camera is now nded just double the equivalent focus of the lens. But »e question arises, from what part of the lens to the ground- 5 ass sal we measure ? This is just the point where the encies of the camera maker, than with any intention to —lead photographers—of stating the focus as measured from the back lens to the ground glass, which is, qf course, always shorter, by an amount varying with the construction lens, than the real or equivalent focus. In answer to ‘above question a formula apparently very definite, has ~Ally been supplied, and the querist has been directed to ita single lens, say a spectacle eye, and having as care- F Y as possible ascertained its focus, compare the size of the zage formed by it on the ground glass of a camera, with he size of the image formed of the same object at the same listance, by the compound lens which is the subject of the • enoivy, and then compute the length of the unknown focus relation between the respective images and the known focus. The rule has been stated thus: divide the nown focal length of the single lens by the fraction repre- ting the linear dimensions of the image formed by the single lens and the combination. Thus; let the focal length ■ of tho single lens equal 12 inches, and the images formed by it and the combination be as 3 to 2 ; then 12-1=8 inches, the equivalent focus of the combination required. Or to at the matter more popularly, proceed thus : suppose the focus of the single lens to be twelve inches, and the image