FOCIMETER. This is an instrument employed by M. Claudet for finding the chemical focus of a lens which has not been properly achromatized.
A number of boards are arranged in a spiral about a horizontal axis, at different distances, and each board is painted white, and inscribed with a black letter. These boards are so placed as to be all visible together from the lens. If, when one of them is in exact visual focus, another is found to come out more distinctly in the photograph, it is evident that the 'visual and chemical foci do not coincide, and the lens should then be rejected as unfit for use, for it will be seen by referring to the article on Depth of Focus, that a lens whose chemical and visual foci do not coincide cannot give such perfect definition as one in which they do, other qualities being equal.
Focus. A focus is either "real" or "virtual." A "real" focus is a point through which an assemblage of rays actually pass. A " virtual" focus one through which their directions if produced forwards or backwards would pass.
A lens is said to have " positive focus " when the focus of a parallel pencil refracted through it is on the same side of it as the origin of light. A concave lens has therefore positive focal length.
A lens is said to have " negative focus " when the focus of a parallel pencil refracted through it is on the opposite side of it to the origin of light. A convex lens has therefore negative focal length.
In the former case the focus of the pencil is " virtual," in the latter case " real." The principal focus of a lens, whether positive or negative, is the focus of a pencil of parallel rays after refraction through it.
The equivalent focal length of a combination of lenses having negative focus, is equal to the focal length of a single lens which, when presented to the same objects, gives an image of the same size as the combination.
FocussING Glass. This is a ma„atiifier used for the purpose of magnifying the image on the ground glass, and enabling the operator to get it into better focus. A good form is that of the Ramsden's Eye Piece of a telescope, which consists of two equal single plano-convex lenses, placed with their plane sides outwards, and at a distance apart equal to twoithirds of the focal leng,th of either. When in use the image on the ground glass must be in the principal focus (or thereabouts) of the eye-piece or focussing glass. It must be remembered that Ramsden's hye Piece is not achromatic.