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2i OPTICAL PROJECTION up. For much long-focus work, an extra condenser (also of longer focus), gives the best result. For all usual exhibition purposes the ordinary double-piano, with the lenses in contact, answers very well; and it is only for sharp detail in the best class of photographic slides, or when it is desirable to utilise the field as far as possible up to the very edge of the disc—as in square or cushion slides— that such optical refinements acquire importance. 11, Practical Points in Condensers.—Condensers are usually made of crown glass, density about 1*516. This answers perfectly for ordinary purposes; but for the highest glass of modern exhibitions, which have to cover large screens of twenty to thirty feet diameter, sometimes at great distances, it is objectionable, as the green colour absorbs light which can ill be spared in such circumstances. Chance's ' optical' flints, of course, leave nothing to be desired in this respect, but condensers so made are very expensive; in some cases, however, they are worth while, as the glass is both more colourless, and the lenses are thinner for the same focus. A more common flint would answer practical purposes, and it is desirable that some colourless glass should be introduced if possible, rather than green crown. This need not make them so very much more expensive ; for while ' optical' flint must be homogeneous, and is usually wanted dense, for a condenser, a perceptible amount of stria is of little practical importance, and density is not required, only colourlessness. It is utterly useless to pay the cost of optical perfection in a condenser for any ordinary purposes. A perceptible bubble in the lens next the slide, however, would be a defect of importance, probably showing as a black spot. It is very likely that many purchasers would reject a colourless lens with perceptible striae, rather than a crown lens which showed none; nevertheless there can be no question that the first would be the better condenser, unless the striae were excessive. The point most commercial condensers chiefly fail in,