FLUORIDE OF SILVER iS soluble compound which does not crystallize. It fuses when heated, and is reduced by exposure to light.
Focau, LINES. When a small oblique pencil is reflected or refracted at a spherical surface, or refracted at a plane surface, the reflected or refracted pencil does not come to a " geometrical focus," or " least circle of aberration," but all the rays composing it pass through two straight lines (or elongated figures of 8), situated in planes at right angles to each other, and called " primary, and secondary focal lines." For instance, suppose the circle at A, situated in a plane perpen dicular to that of the paper, to be the base of a small pencil which has suffered oblique reflection or refraction. Then all the rays composing this pencil will first pass through the " primary focal line" which is perpendicular to the plane of the paper, and after wards through the " secondary focal line" which is in the plane of the paper. The reason of this it would be out of place to discuss
in the present work ; the fact must be taken for granted ; the de monstration of it will be found in Coddington's " Treatise on Optics." If a section be made of the curious solid formed by the pencil between the focal lines, at a point q exactly midway between them, that section will be a circle, and it is the nearest approach that the pencil can have to a focus. This circle is called the " circle of least confusion." The proportions of the figure are, of necessity, greatly exagge rated. The distance between the focal lines is in general very small compared with the distance of either of them from A ; but the distance and also the diameter of the circle of least confusion, increase as the obliquity of the incident pencil increases.