ASCERTAINING THE Focus OF A LENS.
Fuller particulars on determining the focus of a lens may he useful. If a number of parallel rays of light fall upon the sur face of a. double-convex lens, striking it at an angle, they will be refracted or bent towards the centre ray, and the point where these rays meet is the principal focal point. The greater the convexity or out ward curvature of the lens, the more will these rays be bent and the nearer will this point be to the lens. The focal length is the distance from this. point to the optical centre of the lens. The focus of a lens is the nearest point to the lens at which a sharp image can be obtained of a distant object. In the case of a single lens it is sufficient to focus an object. at least 50 ft. away, and to measure with a foot-rule the distance from the ground surface of the focussing screen to the back surface of the lens. In the case of a doublet or compound lens, it may be ascertained in the same way, but it is then generally the " equi valent " and not the " back " focus which is required ; or, in other words, the focus of a single lens which would give an image of the same size at the same distance from the object. This is found with fair accu
racy by the following method : —On a wall stick two strips of white paper, the outer edges of which are exactly 2 ft. apart (or make other marks which can readily be seen on ground glass), and focus these sharply one-fourth the original size, so that on the ground glass they will be 6 in. apart. Then measure the exact distance from wall to screen. A weight attached to a piece of string, and held in the plane of the ground glass, will enable the exact distance to be measured along the floor. This distance has to be multiplied by the ratio or pro portion of image to object, and the pro duct, divided by the ratio plus one squared. Therefore, supposing the dis tance from wall to ground glass is 50 in., the working will be 50 x 4 (the ratio) (4 + 1 squared) = 200 ÷ 25 ---= 8 inches, the required focal length.