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THE PROJECTION MICROSCOPE 187 slides into the body-tube carrying the objective, and must be guarded against any ' flare' or internal reflection from its sides. The effect of such a lens is to lengthen the focus of any objective in any given position; hence its use also increases the working distance. For photographic purposes, the amplifier should be corrected; but for ordinary projection a simple lens is sufficient. It is convenient to have a set of amplifiers of various foci, which give a great range of powers, and of working distances from the screen, enabling very similar results to be obtained under different conditions. The use of an amplifier has another considerable advantage. Ordinary micro-objectives are corrected to a focal length of about 10 inches in England, and 6 or 7 inches on the Con- tinent. If such lenses are used to project an image, say, 25 feet away, besides the mere greater magnification of any error, it is obvious that their ' corrections' can no longer be accurate. It is really wonderful that some lenses should perform, under such trying conditions, so well as they do. But now suppose some given lens is focussed on a slide, so as to project an image at its own proper distance of 6 inches. Our screen however is, we will say, 25 feet away; a distance I have found to give on the average perhaps the best range of general results, though shorter and longer are better for some purposes. We can select a concave amplifier of a focus which, placed in a certain position, will bring the image to a focus on the screen without altering the focus on the object, thus maintaining the 'corrections ' of the objective at nearly their proper value. The late Colonel W. Woodward, U.S., was, I believe, the first to point out this fact, which is of great use in photo-micrography. Any concave lens can be placed somewhere so as to focus a 6-inch or 10-inch focus on the screen, at any longer distance; but practically we are limited to a certain range. I have found generally useful quite a low power, which does this at about four inches behind the objective, and amplifies con-