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 



