Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

ON PROJECTION with a large pin. It is different directly. Because all the rays are straight lines, only the small bundle of rays which the pinhole allows to pass from one point of the slide, can now get to any one point on the screen, and no others can get there. Simply to secure this is all that has been done ; but now it is quite another story, and it will be seen that the bare rays of light, without any lens, do form an image of the slide plainly enough. It will be found that the light will have to be drawn back somewhat from its usual position in the lantern to get the best effect, and especially to make the edges of the picture visible clearly. The reason for this will appear in § 6. It will be under- stood at once how the rays crossing at the pinhole, as in fig. 1, go from the bottom of the slide to the top of the screen, and from the right to the left, so that the image must be in- verted, and we have to place the slide upside down in the lan- tern to make the image come right. And it will also be seen how and why the relative size of the image on the screen depends upon the ratio between the distance of the object, o, from the pinhole, A (fig. 1), and that of the screen-image, I, from the same point. The image before us is but dim, because so few rays can pass through the pinhole to form it. 1 Prick four more holes at equal distances, half-an-inch out from the central hole. i With a mixing jet, an ordinary landscape slide can be made clearly visible on a 12-feet disc, and with a blow-through jet 8 or 9 feet. With oil- lamps a somewhat smaller disc must suffice if the image is to be seen dis- B2 FIG. 1