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188 OPTICAL PROJECTION siderably, being however chiefly valuable for the reason here explained. Such a length of tube also cuts off some of the outer and most indistinct portion of the field. The farther the lens is placed from the objective, the more the image is amplified, the more it is contracted or cut off at the margin, and the less the focus of the objective is altered from what it would be if used alone. We may therefore use the amplifier nearer the objective than its best ' correction' point, for the sake of a larger field ; or farther from it, for the sake of more amplification; in either case at a slight sacrifice of the best performance of the objective. Practically I find the most useful range of adjustment to be an objective-fitting and amplifier-tube which will allow the amplifier to be placed four inches from the screw-threads, or to be pushed in as close as two inches. The last will give as large a field as the objective alone ; at four inches some of the outer portion of the image is cut off, but the rest is more amplified. Now and then it is useful to employ quite a long (six-inch) amplifier-tube, and to use a high-power lens there. The amplification then is very great; but only the central portion of the image reaches the end of such a long tube, which must be carefully guarded against ' flare' by being lined with black velvet. Amplification by Eye-pieces. —Another way of ampli- fying the original image, is to employ an eye-piece, at the end of a tube-fitting of the ordinary length, instead of the short fitting just described. Optically, this method is more perfect than the other, and for photography far preferable, a sharper image being obtained, and a flatter field, with a sharp circular black margin to the disc on the screen. Practically, for ordinary purposes, an eye-piece entails more trouble; but undoubtedly it gives the finest projections. Zeiss's well-known projection eye-pieces have lately made this method familiar, and need no recommendation for photo- graphic purposes ; but they are not suitable for ordinary screen