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THE PARTS OF A LANTERN 31 slide itself, and these scattered rays also diverge more with the greater distance. This is the chief reason why more light is required in working a long way from the screen; though light is also perceptibly absorbed by the atmosphere, especially if it is at all a damp night. Hence it is an important question, how far this loss of light from long focus must be provided for. Some makers recommend several sets of double-combination objectives of very large diameter, at a cost of 9Z. to 10£. apiece! After investigating the matter carefully, both in theory and practice, I have not the slightest hesitation in saying that such costly lenses are a sheer waste of money, of absolutely no benefit to anyone but the seller of the apparatus. Assuming the back conjugate focus of the condenser to be 4 inches (i.e. about 8 inches from the back of the condenser) and the radiant to be |-inch in diameter, nearly all the effective rays can be condensed into an objective of 2-inches diameter, up to 9 inches from the face of the front lens of the condenser. This gives a focus for a double combination of, say, 10 inches, within which there is no need of greater diameter, and no benefit from it. Beyond that a 2-inch lens will begin to lose light, but 2J-inches diameter will carry the same result up to 13 inches; and it is only when we reach distances of 15 inches and upwards that 3-inch lenses are of any real advantage. For such long foci, however, the ' double combination ' is quite unnecessary, as with care in selection, single achromatics can be obtained, which will give quite as good images with far less weight and expense; and perfection should rather be sought in improving this class of lenses, especially in the triplet form described a page or two back; which will also pass more light than the double form at these long ranges. A ' double-combination ' lens of average focus—say 6 to 8 inches—and of most excellent quality, such as will give perfect definition without any stop, is now obtainable for so moderate a sum as II, 10s., including the rack-work mount.