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 



