FLUORINE (Symbol, F; atomic weight, i9i).—A non-metallic element, only recently obtained in a free state. It is an isometric, transparent mineral usually obtained from fluor spar.

Principal a pencil of parallel rays enter a convergent lens, they will be brought together at a point F (Fig. 192) on the other side of the lens. This point is termed the prinatal focal point, and its distance from the lens is termed the focal length. Those rays which pursue a parallel course before they enter the lens are condensed to a point nearer the lens than diverging rays. The rays of light proceeding from very distant objects are parallel, while those from near objects diverge. When the rays diverge from a point, that point is associated with the focus, and the two are termed conjugate foci. The distance between the lens and the object is the anterior or major conjugate, and that existing between the lens and the ground glass the posterior or minor conjugate focus. In Fig. 193 F is the principal focus, and A and B are the conjugate foci. If an object be placed at A it will have its focus at B, and conversely if placed at B, its focus will be at A. It will therefore be seen that the principal focus of the lens never varies, but the length of focus for objects near at hand becomes greater the nearer they are brought to the lens. The relation of the conjugate foci is governed by certain laws. On page 135 will be found a table of conjugate foci, which should be carefully studied.

In dispersing lenses the focus is termed negative and is situated at a point where the rays meet (see Fig. 194).

Actinic the heading Aberration will be found some remarks concerning chromatic aberration, by which it will be seen that unless the lens be rendered achromatic the actinic or chemical focus does not coincide with the visual focus. To make this clearer we have in Fig. 195 the focus of a non-achromatic lens. Its visual focus is at V, while its chemical focus is at C. The ground glass of the camera would be placed at V. and the image focused to the eye, but the chemical focus being at C it would be necessary to push it to that point, in order to get a sharp negative. Most modern lenses are, however, achromatized. The method by which this is performed is explained in Chromatic Aberration under Aberration.

Depth of Focus of a lens is its power of rendering a sharp image upon the plane ground glass screen of objects situated at varying distances. A lens working at full aperture rarely renders near and remote objects perfectly clear at the same time. By using a diaphragm a portion of the rays is cut off, and a much greater depth of focus is obtained, but with the sacrifice of illumination, necessitating increased exposure. (See also Diaphragm.) Equivalent term applied to a com pound or doublet lens. It is the focus or parallel rays entering the lens, and is termed equivalent from being compared with a single lens producing the same size image when placed at the same distance from the object. To be perfectly correct, the point from which this focus should be measured is situated between the diaphragm slot and the back combination. It is, how ever, usually measured from the slot itself, and this measurement will usually be found sufficiently ac curate if an object over 150 yards away be focused on the ground glass and the distance between the diaphragm slot and the screen be measured. Grubb, the well-known optician, gives the following method for obtaining the true equivalent focus of a lens :—" Set the camera before a window upon a flat table, upon which is spread a sheet of white paper. Focus upon some distant object about Iso or 200 feet away. Draw two upright lines on the focusing screen at one inch from the right and left sides respectively. Make the focused image fall on one of these lines, and with a pencil draw a line upon the paper along the side of the camera. Now move the camera so that the image appears on the

other line, and draw another mark alongside the .camera. Take away the camera, and continue these lines with a ruler until they meet to form an angle. Next draw a line across to form a triangle, the line being exactly the same length as the distance between the two lines on the screen. Find the centre of this line, and connect it with the junction of the two angular lines ; this will represent the true equivalent focus of the lens. Example : Lines A A, B B (Fig. 196), are those made on the white paper by the sides of the camera. These lines are extended until they meet at C. E D is the distance of the two pencil lines drawn on the focusing screen. F is the centre of this line and F C the equiv alent focus of the lens." Abney in his instruction book, gives the following method of discovering the true equiv alent focus of the lens : Measure a distance of, say, one hundred feet away from a fixed point, and place a rod at one extremity. From this point measure a line of some forty feet in length at exactly right angles to the first, and place another rod at the other end. Next place the front of the camera containing the lens to be tested exactly over the start ing point of the first line, and level it, the lens being in the direction of the first line. Having marked a central ver tical line on the ground glass with a pencil, focus the first rod accurately so as to fall on the pencil line on the ground glass. Take a picture of the two rods in the usual manner, and measure back as accurate= ly as possible the distance of the centre of the ground glass from the starting point, and also the distance apart of the two images of the rods (at their base) upon the resulting negatives.

Suppose the first measured line A B (Fig. 197) be 149 feet, B D the second line to be 35 feet, A C to be I foot, and E C the distance apart of the two images to be three inches, F being the point where D E cuts C B. Then B D + CE : CB : : CE : CF, which is the equivalent for focal distance. Here CB = 15o feet, BD + CE = 35.25 feet, CE = .25 feet.

150 )( .25 .•.CF = = 1.063ft.

35.25 This gives us the equivalent focal distance, which is the distance of the ground glass from the optical centre of the lens. The thickness of the ground glass having been previously taken, the distance should be set off from its smooth side on to the brass work of the lens. This point, which is the optical centre, should be clear ly marked on the lens, as from it all measurements can be calculated, and the knowledge of its position will be found very useful.

A very simple method of ascertaining the equivalent focus of a doublet lens is to multiply the foci of the two combinations and divide by the sum obtained by adding them together, and subtract the distance of separation, the result being the equivalent focus of the two lenses com bined. For example, we require to know the equivalent focus of a lens, the front combination of which has a focus of io inches and the back of 8 inches, while they are separated from each other by a space of 2 inches, then to x 8 + [( to + 8) — 2] = 8o + 16 = 5 inches, the true equiv alent focus.

After a certain distance all objects will be in focus. The following table* will be found useful for many purposes. It shows at a glance the distance beyond which all ,objects will be sharply defined, and it is a good plan to mark the stops with this distance, so that if a subject be taken requiring a particular stop to obtain correct focus, and it is discovered that this stop. requires too long an exposure, the subject should not be taken, and thus a plate will be saved For detective camera work this table is very useful.