OPTICAL CENTRE OF CONIPOU ND LENSES.
In doublet lenses of rectilinear con struction, and also in must symmetrical combinations, the optical centre of the lens is situated practically at the dia phragm. and measurements from the latter to the ground glass will give a sufficiently accurate estimate of the equivalent focus for all ordinary purposes. In other forms of compound lenses which are not symmetrical, the optical centre may vary considerably in position, being either in front of or behind the diaphragm. Fortunately, although it 13 difficult to ar is also the case with a double concave lens. With plano-concave and plano-convex lenses it is impossible to draw the parallel radii, since there is only one curve. The optical centre is therefore, as a little con sideration will show, on the curved sur face of the lens, where it is cut by the axis (see Fig. 495). A concavo-eonvex lens has its optical centre outside and in front, as in Fig. 496 : whilst a meniscus lens has its centre outside and behind (Fig. 497). It will be seen that it is an easy matter to calculate the optical centre rive at the position of the optical centre by calculation in such cases, there is a practical method available of great sim plicity. This consists of focussing the lens
being tested on a distant object, and selecting a thin spectacle lens which will give an image of precisely the same size from the same standpoint. This is readily done by the optician, who usually keeps a large number of different kinds of speetaele lenses at hand for testing purposes. If, then, the distance is measured from the edge of the spectacle lens, which is found to give an image of the desired size, to the ground glass, this will be equal to the equivalent focus of the compound lens under examination, and it is easy to mark the optical centre of the latter on the lens mount. As a matter of fact, every compound lens has two optical centres, also called equivalent.
or nodal planes, according to the direction in which the lens is turned. Most lenses, however, are only intended to be used one way round, and it is only necessary to ascertain the optical centre of the ob jective when pointed in that direction.