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6 OPTICAL PROJECTION some fixed distance related to the other distance, according to the refractive power of the glass. It is accordingly called the conjugate focus of the point/. Here we have exactly what we wanted to get—a brighter image. To make it more clear, take both the slide and the condensers also out of the lantern, leaving only the perforated tinfoil with its five holes, and let the object be the light in the lantern itself, because its images will be both brighter, and will stand out obviously apart upon the screen. Now take a lens ! in the hand, and hold it close to the tinfoil. At once the outside images will be bent in towards the centre one, and a position will be readily found in which all are exactly made to fall on one spot. Finding that position, and placing the lens there, is ' focussing ' the lens. This image is obviously five times as bright as the original one, and if now the lens can be supported in that position, we may prick any number of holes, and break away the tinfoil altogether, and it makes no difference except to further brighten the image; because in the same way all the images formed by every ray are bent in to the same point, and unite their brightness .in one. This is the only action of a projecting lens. People talk, and even write, as if it formed the image, or inverted the image ; but it does nothing of the kind. The rays themselves form the images; and their crossing at the place where the lens is, inverts the image; and all the lens does is to bend into one spot the rays (forming images) which fall upon its surface, and so to combine countless faint and jumbled images into one bright and clear one. That is the true nature of Optical Projection. This much being experimentally realised, several very important principles at once become clear. First of all, it 1 This lens must be of somewhat longer focus than the distance from the light to the tinfoil, or the light must be pushed up to the front so as to malca ihat distance less than the lens focus.