fice here to say that the focal length of the lens-the distance from its effective aperture at which parallel rays are made to meet in a point or disc of light-is the most vital single fact to be known about it, since focal length largely determines the utility and value of the objective. It absolutely determines the kind of camera with which the lens may be used, as the following illustration will show:

A lens of 21⁄2 inches' focal length treats all rays emanating from luminous objects 30 feet or more distant, as if they were parallel and projected from infinity,

sensibly sharp definition at the same instant, of several objects at different distances within one-half mile of the camera. To do this, the aperture must be reduced, making the lens much slower.

Moreover, focal length is one term of the ratio involving "effective aperture,” upon which, almost entirely, depends the working speed of the lens, and consequently its adaptability for a variety of valuable uses. Since at best only one eighty-thousandth of the luminous rays from a reflecting object can enter the lens, it is obvious that the focal-aperture

ratio should be as great as possible without impairing the performance of the objective; but every decrease of the aperture for the sake of depth of focus, geometrically diminishes its light-gathering and transmitting capacity, making it slower.

Coming now to the camera, it is fair to assert that for universal application in the hands of an amateur with a serious purpose, nothing possibly can be better than a five by seven inch folding "box," with as long a bellows as the buyer can afford, and fitted with all the devices for shifting the back and front; a ground-glass screen for focusing; a level and a plumb, for adjustments of position; and a rack-and-pinion movement for fine focusing of the image. The camera should be adapted for use either with glass plates or cut films, either separately or in "film packs" of a dozen. units. The workmanship of all the cameras of good grade is most excellent; hence it is not essential to pay a fancy price for much lacquered brasswork. A plain, solid box with a stout bellows and the movements noted, may be had at a moderate price, and will last a lifetime if properly cared for.

We insist on as long a bellows as the buyer can afford. The novice does not appreciate the wisdom of this counsel; but after he has tried to do high-class book illustrating with a short-focus, wide-angle lens and a short bellows, he

DAY'S DYING GLOW.

A decorative theme based on the "reverse curve" traversing the foreground and ending with the dome-shaped foliage of the tree at the crest of the hill. Observe the abundance of subordinate detail in the foreground, and the simple beauty of the luminous distance against which, in tender silhouette, stands the principal mass of the picture.

value of his outfit, at small additional outlay.

Suppose an engineer wishes to study at leisure the details of a novel truss or car truck. He will want as large an image of the device as his plate will hold; and if he possesses a long bellows, it will be possible for him to copy important details on a half-scale; whereas, with a fixed-focus box camera, his picture would be so small as to require a magnifier to resolve details.

The Anastigmat Lens

At the present time, all standard view and hand cameras are sold with common

stock lenses will answer for a time; but the ambition of every photographer, novice or adept, is to own an "anastigmat." Hence it will be well for the beginner, if his purse will permit, to buy a "Jena glass" objective at the outset. The question then arises in his mind-Which of several hundreds all "The Best," really is best? And this compels us to dwell in some detail on the differences between the new and the old types of lenses.

Avoiding the technicalities of the subject, we may say broadly, that the old

to an approximate focus wave lengths representing two colors at or near the ends of the spectrum, and only two at the same time and in the same lens. The colors selected for this collection were, the one having the greatest ocular intensity, and the one of greatest chemical activity. By combining in a tube at precisely the correct positions two of these "simple achromatic" lenses, a doublet was produced, having sufficient elements of curvature, thickness, and separation to enable the opticians to correct some of the numerous errors of sphericity, but always at the expense of aperture and speed, or of angular field, or of marginal definition. With the old glasses, it was a physical impossibility to obtain a flat field without reducing the aperture; and even then the angular extent of the field was limited to a few degrees. It was impossible, mathematically and practically, to combine the old glasses in such a way as to eliminate "astigmatism"-that is, the fault in a lens whereby adjacent pencils of light coming from the same source fall in different focal planes. This is a very serious error, and it seemed to be incurable.

Schott and Abbé, however, discovered that certain silicates of barium afforded glasses in which refraction and dispersion varied inversely and abnormally,

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optical glasses available prior to the discoveries of Schott of Jena, made it possible to form lenses capable of bringing

complying with the theoretical specifications furnished by Dr. Abbé and Prof. von Seidel. Then, and only then, it be

came possible to construct lenses free from all the gross errors of the old types, and approximately devoid of all spherical and chromatic aberrations, both primary and secondary. The new glasses, however, do not render the old useless, since the theory of modern construction requires not only abnormal but also normal pairs of glasses; and the perfect anastigmat necessarily involves the combination of old and new glass components, to effect all the necessary corrections.

Prime among the advantages of the new construction are these: Absence of curvature and of astigmatism over a very

The anastigmats are offered in endless variety and at all prices, depending on focal length and aperture—some convertible, some symmetrical, and some unsymmetrical. The ultra-rapid and the extreme wide-angle lenses are distinctly special objectives; and unless the worker requires them, he would better select a more generally useful objective, working at moderate speed, and capable of being used either as a doublet or as a long-focus single. This means a symmetrical anastigmat. For a 5-by-7-inch camera, an 8-inch lens should be chosen, assuming that the bellows is long enough

wide area; almost complete correction of all varieties of spherical error; approximately perfect elimination of errors of color; and the attainment of all these advantages without diminution of aperture-indeed, so much to the contrary, that all these merits now exist in lenses having the enormous focal-aperture of F/3.6.

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It is needless to say that the anastigmat lens is not a "fad." It is a scientific achievement of surpassing magnitude; and there is no sort of comparison between the new and the old that does not show the old to be vastly inferior. Still, where the purse rules in favor of the old type, and the matter is not open for debate, it is comforting know that the common stock lenses of the present day are much better than the precision objectives of the time before Frauenhofer. In fact, the uncritical amateur may never become painfully conscious of the limitations of his cheap lens unless he happen to do work in comparison with that of an anastigmat. Use the best tools you can afford. That is the only rule.

to accommodate the 16-inch single, and give three or four inches for manœuvering at closer range. Such a lens, with an aperture ratio of F/6.3 to F/6.8 will be of universal utility.

The subject of lenses is much too complicated for brief discussion. If any reader desires to be fully informed on the technical and practical points involved, he can do no better than to read carefully Prof. Sylvanus Thompson's translation of Dr. Lummer's "Photographic Optics," in which the theory of aberrations and their corrections is fully developed.

Focus and Adjustment

Assuming that the worker is provided with such an outfit as we have described, and with the necessary plate-holders, tripod, focusing cloth, and dark-room paraphernalia, the next step is to understand how to use them advantageously. It is well to make sure, first, that the plate-holders, when in place, bring the surface of the sensitive plate into exact coincidence with the image as seen on the focusing glass. To insure this es

sential, it is well to focus carefully with a magnifier some object of bold outlines, noting with care its features on the ground glass, and then exposing a plate without altering the position of the camera. If, on development, the image is found to correspond accurately with the object as focused on the screen, it is safe to approve the "register."

It is of almost equal importance that the lens shall be mounted on a front board accurately parallel with the plane of the plate. Any fault in this adjustment causes trouble when fine work is attempted. The finer the lens you use, the more vital it is to have the plate at a right angle with the axis of the objective. Incidentally, users of anastigmats ought to focus with the aid of magnifying glasses if they wish to obtain the best results. Carelessness in this particular leads to much disappointment.

Plates or Films?

The decision of the issue between plates and films resolves itself into a problem of convenience. The old "roll film" is no longer imperative, since cut sizes are readily obtainable, and packs of a dozen exposures may be had for adaptation to almost all cameras. The use of the film-pack enables the worker to focus his view on the ground glass just as with glass plates and at the same time gives him the considerable advan

tage of the light celluloid. For the tourist, films are the proper thing; but for the indoor worker, glass plates cannot be surpassed.

For all work likely to involve manipulation of the resulting negative, or special methods of printing, plates will be prescribed. For out-of-door work, where "non-halation" quality and extreme portability are desired, the filmpack should be chosen. Plates are cheaper; and during the apprentice stage of the beginner's career, he may well use them almost exclusively. Of great value, too, is the fact that plates are supplied with emulsions suited for a wide range of purposes-from the very slow emulsions to the most rapid color-sensitized varieties.

If copying is to be done, select a slow emulsion either ordinary or "ortho," according to the problem in hand. For interior work, rapid non-halation plates are all but essential, to avoid the spreading of high lights and incidental troubles.

For landscape work requiring good rendering of "values," one must use isochromatic or orthochromatic plates, in either case with appropriate selective color screens.

The beginner can afford to learn most of his preliminary lessons with the cheap. ordinary plates, reserving for special occasions the higher-priced "orthos" and non-halation brands.