A photographic objective is almost invariably a combination of a number of positive and negative lenses of various forms and varieties of glass, so arranged that the errors, or "aberrations," of one lens or set of lenses is counterbalanced or "compensated" by opposite errors produced by the other lenses. Negative lenses are introduced for this purpose, as their errors are generally of an opposite nature to those of positive lenses. The optician attempts to thus compensate all aberrations, but when the perfect balancing of the errors proves impossible he makes use of a " stop " or "diaphragm," which is virtually a perforated screen with which very erratic and quite uncontrollable light rays can be obstructed or " cut out," while those that submit to control are allowed to pass. Such a diaphragm will eliminate the effects of most forms of aberration if the aperture is small enough, therefore it affords the easiest and cheapest means of making a lens produce passable results. Even a single uncorrected lens will do good work with a small stop put in just the right position; but, with any lens, the use of a small stop aperture reduces the quantity of light that passes the lens, which becomes " slow," or of low " intensity," and requires long exposures. If great rapidity and short exposure is necessary, the use of a small stop is impossible, and the optician has to use his utmost skill in applying the principle of compensation. He cannot do altogether without the stop, but he must place as little reliance upon it as possible for purposes of correction; hence the difficulty of making very rapid perfectly acting lenses and their relatively high cost.