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

ON PROJECTION g was all that could be desired. The fact must be, therefore, that the more luminous rays from the edge of the slide, while they fell upon and got through the lens, did not get through the pinhole until we drew back the light. Let us investigate this further. With the slide in the lantern, leave the lens in its place; but this time remove the condensers instead. The slide itself is now very nearly as brightly illuminated as before; but only the centre of the picture is at all bright upon the screen, and we have again the dark margin, all the more striking by contrast with the bright centre. Considering this, we begin to understand the state of the case. Our slide is very greatly transparent. (It is not wholly so, or there could be no picture of it, nor FIG. 5 could we see it.) Now, any imperfectly transparent object scatters light falling upon it, and in this way sends out some rays in all directions open in space. But the greater part of the luminous rays which strike on it do go straight on from the light in the lantern, or continue diverging ; and as regards far the greater part of the luminous rays, therefore, the case stands as in fig. 5. Those passing from the radiant, B, through the outer part of the slide, s, pass outside the lens, L, and give no light to the final image ; the lens only picks up from the margin of the slide the few comparatively dim ' scattered ' rays. On the other hand, a central cone of bright direct rays does get to the lens, L, and forms a good image of the central portion of the slide. The properties of a lens already indicated, suggested long