_The New Method._—There is another process of straightening barrels which was explained by a writer in a scientific paper a few years ago, which is termed “straightening by the shade,” and by this method barrels can be straightened with a greater degree of precision than by any other known process. The principle is something like this: If we examine a plane mirror for the purpose of ascertaining whether its reflecting surface is a true plane, we cause objects to be reflected from it to the eye at small angles of incidence. If under these circumstances every part of the mirror gives an image true to nature, he pronounces it perfect; for the slightest deviation from a true plane would cause a manifest distortion of the image. In the process of straightening barrels by the shade, crooks in the bore are detected upon the same principle. The internal surface of the barrel is a mirror, and whatever objects are reflected to the eye from any portion of it that lies beyond a certain distance, will be reflected under very small angles of incidence. As the interior surface of the barrel is not a plane mirror, the reflected image will not be true to nature. If the bore be straight, the image will have a normal distortion, which is due to the transverse or cylindrical curvature of the mirror; while if there be longitudinal flexures or crooks, there will be an abnormal distortion of the image, which will reveal the defect. [Illustration: FIGURE 31.] When the eye looks into a gun barrel the interior surface appears to be spread out into a plane circular disk, as far from the eye as the other end of the barrel. Through the centre of this disk is a circular orifice, and surrounding this at equal distances from it and from each other, respectively, are several well-defined concentric circles, dividing the disk into as many bright concentric rings, each of an apparent breadth, precisely equal to the diameter of the central orifice which is the other end of the bore as seen by direct vision. The several concentric circles are so many images of the end of the bore reflected to the eye from different points along its length. The first of these circles, or that nearest the central orifice, is an image formed by light once reflected. The second, third, fourth, etc., respectively, are images formed by light reflected two, three, four, times, etc. In order to see how these images are formed, and to find their respective points of location in the bore, consider that a ray of light from each point in the end of the calibre, as shown at _a_, Fig. 31, may pass to same point _b_, on the other side of the bore, and be thence reflected to the eye, thus forming at _b_, an image at the end of the bore, of one reflection. Another ray from the same point may pursue the route _a_, _c_, _d_, _e_, forming an image at _d_ of two reflections. Another ray may take the route, _a_, _f_, _g_, _h_, _e_, forming an image at _h_ by