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2 OPTICAL PROJECTION less discussion, the almost self-evident fact that objects are visible by means of rays of light which they either emit of themselves (as in a gas-flame) or reflect back from some other luminous source (as in an object lighted up by the gas-flame), all we need assume here is that these rays are sent out in straight lines, in all directions which are open in space, continuing to travel in straight lines so long as they traverse the same medium, as the air, for instance. This is a familiar fact of experience, as shown by sunbeams or rays from the lantern. Now it is the fact that every ray which thus proceeds from any point of any object, really forms an image of that point of the object upon any surface on which it falls; and it should be clearly understood that if the mere bare rays of light, by themselves alone, had not this power of forming images which is here affirmed, all the lenses in the world could never do it. Not being very self-evident, this fact should be realised by experiment. 3. All Rays form Images.—As the reader will possess a lantern of some sort, this will afford the readiest demonstra- tion. Place a slide in the stage, choosing one which has some well-marked and large features, and is tolerably trans- parent in the rest, and throw the image on the screen as usual; such as is seen every day, and which is supposed to be formed by the lens in front of the lantern. We now take that lens out, and there certainly appears to be no image upon the screen, though the rays from the illuminated slide stream out to it in plenty, and the screen is lighted up well enough, and there may be signs of colour if it is a coloured slide. But let us consider a moment. The rays of light can go now from every point of the slide alike, to any one point on the screen; therefore what we see now at any point on the screen will be the total of these superposed images, of all the points in the slide, from which no one stands out particularly. But we can stop all that easily. Cover over the empty brass front with a sheet of tinfoil, and in the centre of this prick a hole