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March 27, 1868.] THE PHOTOGRAPHIC NEWS. 147 of which is still but imperfectly understood. What is true in the chemistry of photography is true in relation to its optics. Wo do not mean to assert that opticians have worked by rule-of-thumb rather than by mathematical knowledge, although that position might possibly, in some degree, be justified ; but we mean that many photographers have acquired some knowledge of the use of lenses, and of the application of special lenses to special purposes, with out the slightest knowledge of their construction or of the reason why a lens excellent for one purpose failed in another, or of the principles upon which their selection of lenses for various uses should be based. Dr. V an Monckhoven believes, —and, we fear, with much reason—that the knowledge of how to use a lens is “ a thing about which ninety-nine photo graphers out of a hundred arc in ignorance,” and adds, that “ to use a lens improperly is to produce portraits false in per spective, buildings and houses falling into the street,” &c. It is tolerably certain that more ignorance prevails in regard to the optics of photography, even amongst cultivated photo graphers, than in regard to any other of the multiform branches of knowledge requisite to constitute a really good photographer, and that any work which may aid to dispel that ignorance should be hailed with welcome by the photo graphic community. Dr. Van Monckhoven’s treatise on photographic optics was issued in French about two years ago, and is now intro duced by Mr. Hardwicke to photographers in an English dress. No translator’s or editor’s name is appended to the work; but, with the exception of the insertion, in one in stance, of a foot-note by the Editor, attempting, but erro neously, to correct the author, the work seems very well done. We need scarcely remark that the task of producing a work on optics for photographers was an exceedingly diffi cult one. It was necessary to preserve scientific accuracy, and it was not less necessary to deal with the subject in a popular manner; and in attempting both, the work ran the risk of being pronounced superficial by scholars, and abstruse by the unlearned. Nevertheless, without sacrificing pre cision—some few doubtful positions excepted — Dr. Van Monckhoven has endeavoured to write a popular book, and has produced an addition to the text books of the art which every photographer ought to possess. The work is divided into two books : the first treating of optics and lenses, as applying to the ordinary practice of photography ; and the second of the apparatus and processes employed in enlarging. Th* early chapters of the first part are devoted to a re statement of the known laws of light and of optics gene rally, treated with especial relation to photography, and furnishes a succinct account of the history, quality, and con struction of the various lenses now in use. The various aberrations—‘spherical aberration, chromatic aberration, curvature of field, distortion, and astigmation—are carefully considered, in connection with their causes and cure, and the relative degree in which they each prevail in different kinds of lenses, the copious use of diagrams tending materially to aid the student in a perfect understanding of the author. After treating fully of the aberrations of lenses, the author proceeds to the classification of lenses under two distinct heads, the aplanatic and the non-aplanatic, the former giving sharp images over a small field with their entire aperture, and the latter, requiring stopping down, giving sharp images over a more extended field. It is in the treatment of this part of his subject that the author lays himself most open to the charge ot shackling himself with theories the truth of which is not borne out in practice. Many of the new lenses which have been introduced of late, and which he classifies as non-aplanatic, he utterly condemns as a retro grade rather than an advancing step in photographic optics. All single combinations come under this sweeping condem nation, as well as some other lenses which possess a high popularity amongst photographers. The reason alleged is as follows :— The practice of photography has established that, when the image at the focus of a lens is wanting in intensity, the photo graphic reproduction of this imago is itself wanting in relief, the foregrounds being too black, the objects situated in the horizon confounded with the sky, and the clouds in the sky replaced by a plain ground of uniform tint; the proof, in a word, is wanting in aerial perspective, and, if it be a portrait, in vigour and relief. For, to give sharp images, non-aplanatic objectives require very small diaphragms, and generally of fromto f being their focal length: hence an insufficient intensity in the imago and the defect which we have just pointed out. But, exempt from distortion, and including a considerable angle, they are useful in some special cases, such as the reproduction of cartes, buildings situated a very short distance, or landscapes and buildings strongly illuminated by a powerful sun. Aplanatie objectives, under the head of which rank the triplet, include a loss angle, but do not require diaphragms exceed ing 3; and therefore they give more artistic photographic proofs, in which the foregrounds and the horizons are well brought out, and the skies have clouds. If the light is insufficient, they are employed with a larger diaphragm ; and the sharpness of the image is not destroyed as with non-aplanatic objectives, but only limited to a smaller field. They can be used for portraits in the open air, groups, and animated scones, with their entire aperture— an advantage which is invaluable in practice. Lastly, the angle they include, being between 50 and 60 degrees, is more than suffi cient, because, if this angle is more considerable, the effect of the perspective is doubtless more astonishing than agreeable. In our opinion, therefore, the use of non-aplanatic objectives— such as the single lens, the globe lens, Mr. Ross's doublet, and that of A. de Steinheil—should be abandoned (except in some special cases, which we have enumerated above) for that of aplanatic objectives, among which the triplet is the best, as being free from distortion. Whilst fully endorsing the author's views as to the ex cellence of the triple, we take exception to his wholesale condemnation of many of the lenses the abandonment of which ho recommends. For landscape purposes, many of our best practical photographers prefer the use of a good ordinary single lens; the wide-angle-single lens, and the rapid stereoscopic single lens of Dallmeyer, are justly regarded as amongst the most useful instruments placed in the hands of photographers; and the reputation of the doublet of Ross is justified by the numerous admirable photographs produced by it, and brought from time to time under the attention of photographers. The author’s position, so far as it is confined to a recommendation to the use of lenses giving good definition with wide apertures, wo most cordially support, and agree with him that lenses requiring small stops to secure definition should be abandoned, as, besides the protracted exposure involved, the pictures pro duced are flat, tame, and unsatisfactory. It is only to his sweeping condemnation of all lenses alike which come with in his theoretical classification as non-aplanatic that we take exception. But whilst strongly expressing his leaning, on theoretical grounds, to the lenses he classes as aplanatic, the author is strictly' fair and just in his description and treatment of the various lenses in detail, dealing with the subject in a spirit of scientific impartiality which is highly commendable. The practical portions of the book will be read by photo graphers with the greatest interest. Here is an example relating to a subject to which the attention of photographers has of late years been much directed, but which is by no means yet exhausted :— Depth of Focus.—Depth of focus is the property of lenses of giving a clear image in planes of which the distance is unequal. It follows from this that the ground glass placed at the focus of a lens may be moved to a very slight extent without the imago sensibly losing its sharpness. To prove this experimentally, bring a camera furnished with a single combination to bear on a landscape. Bring to a focus the objects farthest off. We shall soon remark two things : the first is, that the ground glass can be advanced or withdrawn to a small extent without the sharpness of the image.of a fixed plane of the landscape sensibly changing; the second is, that if we bring to a focus the most distant plane, many other nearer planes will still be in focus. The same experiment can be made with an opera-glass, which may be regulated for distant objects so as that they may bo seen
