on the ground glass and photographed, the negative will not be sharp. The reason for this is as follows. When light is passed through a prism, in addition to the bending of the rays as is shown in Fig. 8b., they are separated into the various colors of which the white light is composed. Each of the colors have a different wave length, red and yellow having the longest come at one end of the scale, and the violet and blue with the shortest wave lengths are at the other (Fig 10a.). Furthermore, the yellow rays are the "visual" rays or the ones by which we see the object on the ground glass, and the violet rays are the "chemical" rays or the ones which effect the sensitive film, so that when we focus such a meniscus lens, the ground glass will be in a plane of the yellow rays back of the plane of the violet and blue rays (see Fig. 10a, C. P. and V. P.

It is not possible to make a lens of a single piece of glass that will bring the yellow and violet rays into the same plane. This

Figure 10. a. A ray of light separated by a prism into the principal colors of which it is composed; v. p., the visual plane and c. p., the chemical plane. b. A negative lens designed from two prisms placed tip to tip, showing the direction a ray of light is bent.

fault is known as chromatic aberration. These lenses are used, however, in some of the cheapest fixed focus box cameras by placing the lens so that the chemical rays will focus on the plate or film.

If a lens is designed from two prisms placed with the points together instead of the bases, the rays of light would be bent out or diverge instead of being bent in or converging as with the latter (Fig 10b.). The converging lens is also called a positive lens and the diverging lens a negative lens.

By using two kinds of glass of different density and combining a converging and diverging lens (Fig. 11a.), it is possible to bring the focus of many of the visual and chemical rays into the same plane. This type of lens is known as an achromatic meniscus. While this lens is a decided advance over the simple meniscus lens, it is not possible to control the rays of light at the edges

of the lens, so that it is necessary to cut out these rays with a stop, and use only those rays coming through the central portion. If we photograph an object with straight lines, such as a window, placing the stop behind the achromatic lens, the lines at the edges of the window in the photograph will curve outward (Fig. 12a.) If the stop is placed at the front of the lens, the edges will curve inward. (Fig. 12b.). By combining two of these lenses with the concave surfaces toward each other and placing the stop between the two, this defect will be overcome and the lines will be straight in the photograph (Fig. 12c.) Such a lens is known as a rectilinear lens (Fig. 11b.) More of the marginal rays can be controlled in a rectilinear than in an achromatic lens, and a stop with a larger opening may be used, which admits between three and four times as much light as the single lens. The rectilinear lens, while satisfactory for many kinds of work,

Figure 11. a. An achromatic lens, consisting of a positive and negative component; b. Construction of a rapid rectilinear lens; c. Diagram illustrating curvature of field of rectilinear lens.

still has many defects. The chromatic aberration is not entirely eliminated. If the image on the ground glass is focused sharply at the center, the edges of the field will not be sharp, showing that it is more or less saucer-shaped (Fig. 11c.). This is known as curvature of the field. In addition to this, some of the marginal rays cross those entering through the central portion of the lens and fall on the plate or film at an angle causing a slight blurring of the image. This defect common to the rectilinear lens, is spherical aberration and can only be corrected by using a smaller stop, eliminating the marginal rays and increasing the exposure. In photographing geometric figures or objects with both vertical and horizontal lines with a rectilinear lens, only one series of lines can be sharply focused, due to astigmatism.

Up to about 30 years ago, only two kinds of glass were available for making lenses: crown glass and flint glass. These were

of different density and therefore each would bend the rays at different angles. Positive lenses or elements were usually made of crown glass, and the negative elements of flint glass. Only a certain degree of skill was required to reduce the principal defects, curvature of the field, chromatic and spherical aberration and astigmatism to the limit obtainable with these glasses.

About this time, a new kind of glass was produced which could be made in different degrees of density, and lenses could be made in which the above mentioned defects could be practically eliminated. This is the Jena glass from which the anastigmat lenses are made. These lenses could be further corrected for certain other faults which are essential in certain critical lines, but of little importance to the ordinary workers.

In the anastigmat lens, it is also possible to bring the light rays entering near the margin of the lens into the same plane as

Figure 12.

a

b

с

a. Diagram of a window sash showing distortion with the stop behind a meniscus lens; b. The same with the stop in front; Lack of distortion when made with a rectilinear lens.

C.

those nearer the center, so that it is not necessary to "stop" them out. For this reason, from 40 to 300 per cent. more light, depending upon the construction, will pass through these lenses than through the rectilinear lens. While the anastigmat lens is comparatively free from optical faults, it is only by using great care in the construction that the defects found in other types of other lenses can be eliminated. The lens glass will vary in density with the amount of heat used in fusing it, and the lens formula must be computed for each lot of glass. The elements are ground and polished to within at least one thirty-thousandths of an inch. This grinding has to be done slowly so as not to heat the glass and cause it to expand. Anastigmat lenses may have from three to 10 glasses in their construction (see Fig. 13) and it is not hard to see why they may cost from five to 10 times as much as rectilinear lenses.

The size of a photographic lens is based on what is known as the focal length. This is approximately the distance from the

ground glass or film to the center of the lens, when it is focused on an object at a distance of 100 feet. Cameras for general work are usually fitted with a lens, the focal length of which is equal to, or slightly shorter, than the diagonal of their plates or films: for example, a 34 x 4 camera with a diagonal of 5.3 inches will have a lens of about 5 inches. Fig. 14 gives the diagonals and focal lengths of lenses for three common sizes of cameras. A lens of shorter focal length will give an exaggerated prospective; that is, nearby objects will appear too large and distant objects will be proportionately small. A lens of a focal length longer than the diagonal of the plate will often give a better prospective, but would be too large to fit the average small hand camera. It is also necessary in photographing large objects,

Figure 13.

Diagrams of the construction of anastigmat lenses, with from three to ten glasses.

such as buildings, to stand further away to include as much in the view as is done with the shorter focus lens.

Lenses are provided with stops to cut down the size of the aperture. There are two systems in general use, both of which are based on the focal length of the lens. In what is known as the "f" system, the diameter of the opening is a fractional part of the focal length; that is, if the largest stop or opening of an eight inch lens is one inch in diameter (one-eighth of the focal length), it would be marked "f.8" which also indicates the speed of the lens and would be called an f.8 lens. The diameter of each of the smaller stops would be indicated by their fractional value as f.11 (one-eleventh), f.16 (one-sixteenth), etc.

In the other system, called the uniform system (U.S.), the equivalent of f.8 is designated as 4, and the number for each smaller stop is twice that of the preceding and indicates that the

exposure should be doubled. This system is generally used with rapid rectilinear lenses, while stops of anastigmats are usually marked with the f. system. The relative stop values of these systems are given below.

F. System....
U. S. System..

Anastigmats are frequently termed "fast lenses" and the novice may get a wrong impression of what is actually implied by this term. The amount of light which passes through a given sized opening of a meniscus lens, say stop f.16, which is usually the largest stop of this type of lens, is exactly the same as will pass through an anastigmat at f.16. The exposure would be the same for either lens, as the amount of light that can enter to affect the film depends simply upon the relative area of the lens opening. As has been stated, the largest stop of the single lens is usually

Figure 14. Three sizes of cameras, their plate diagonals and the focal length of lenses often used on each.

f.16; that of the rectilinear lens is f.8, twice the diameter of the former, and thus admitting four times as much light. The largest aperture or stop of the anastigmat may be larger than f.2, or the diameter of the lens opening more than one-half the focal length of the lens.

Anastigmats fitted to hand cameras, however, rarely have an opening of more than f.6.3, which admits a little over 60 per cent. more light than the rectilinear lens.

In using these large apertures, or in other words, giving a short exposure with the lens wide open, we lose something that is of extreme importance in scientific photography; the depth of focus. The depth of focus is the degree of sharpness of objects in different planes, or distant and nearby objects in the view that we wish to photograph.

This is a fixed quality in all lenses regardless of the type. Depth of focus depends upon two things; the size or focal length of the lens, and the size of the stop used. The longer the focal length, the less depth; the larger the stop, the less depth.