CHEMICAL FOCUS.—A lens possesses two foci, the visual and the chemical. Before the many improvements in optical instruments were made these two foci differed ; in some cases very considerably, so that it was necessary to determine the difference, and after focusing on the screen at the visual focus, to shift it to the chemical focus determined by photographic surfaces. In a good lens, however, these two foci should coincide. To test for chemical focus, place a few yards from the 'camera three cards with sharply defined letters on them, as in Fig. 105. Focus with a magnifying glass on the centre card, and upon developing carefully examine the image with a magnifying glass. If the centre card is the sharpest of the three, the lens has been properly corrected ; but if the first card is the sharpest, the chemical focus is shorter than the visual. It will therefore be necessary to slightly push in the camera before exposure. If on the other hand the letters on the third card are the clearest defined, the defect is of an opposite character, the lens has been over-corrected and has a back focus.
Traill Taylor gives the following method of visually testing for over or under-correction : " Bring the lens to be examined into a slightly darkened room in which there is a gas light burning, and retreating several feet from it hold up the lens so as to form an image of this light in the eye of the observer. The image must, however, be examined through an eye-piece of any good construction. At the point where the image is sharpest there is but little color, but by bringing the portrait lens a little nearer, the flame, if the lens be properly corrected, is seen to be surrounded with a claret fringe, while on removing it to a greater distance than distinct definition, the light is fringed with green, proving that the blue and yellow rays are combined, and, as a consequence, that the chemical and visual foci do coincide.