CENTRING OF LENSES When a lens is correctly centred the axes of all its surfaces are in a straight line ; otherwise, good definition cannot be obtained. Faulty centring gives a distortion to the image similar to that of astigmatism or coma, and is easily detected by fixing the lens upon a camera or optical bench and focusing a brightly illuminated object, such as a small lamp flame or mercury bulb. The position of the image is carefully noted, and the lens gently rotated in its flange, when, if the centring be perfect, the image will remain quite stationary, but if not it will move in more or less of a circular direction upon the screen. The fault may be in the flange, or rather the body ring of the lens tube, or it may be in the thread upon the lens cells, in the cementing of the separate components, or the surfaces of one or more of the single glasses of the combina tion may not be correctly adjusted. In any case, it is a matter for an experienced optical work man to put right.
The centring of the actual components of the lens is a simple process, but one requiring con siderable skill. When a lens leaves the polisher's
hands it has a more or less rough edge, and is a little larger than the cell it is destined to fit. It then goes into the hands of a workman for " edging " and " centring," which operations are performed simultaneously. The lens is stuck with pitch upon a revolving chuck, and a gas flame is so adjusted that two images of it are seen by reflection from the inner and outer surfaces of the lens. These will be found to " wobble " as the lathe head rotates, and the workman slides the lens upon the soft pitch until they are quite steady. The pitch is then allowed to set and the edge of the lens is ground to correct size upon the lathe by the application of a copper plate fed with wet emery powder. A similar process is gone through after cementing two or more glasses together while the balsam is suffi ciently soft to allow the surfaces to be moved upon each other.