54 THE EARLIEST STEPS IN THE INVENTION OF 



(does so affect it). So a boy can appear a giant, a man seem a 

 mountain, and in any size of angle whatever, just as we can see 

 a man under so large an angle like a mountain and as near as we 

 desire. So a small army might seem very large, and though far 

 away appear near, and conversely : so too we could make sun, 

 moon, and stars apparently descend herei below, and similarly appear 

 above the heads of our enemies, and many other similar marvels 

 could be brought to pass, so that the ignorant mortal mind could 

 not endure the truth." (Opus Ma jus, Part V). 



''And what is causally manifest with regard to double refraction 

 we can verify in many ways by the results of experiment. For if 

 anyone holds a crystal ball or a round urinal flask filled with waler 

 in the strong rays of the sun, standing by a window in face of the 



Fig 2. 



rays, he will find a point in the air between himself and the flask 

 at which point, if any easily combustible substance is placed, it 

 will catch fire and burn, which would be impossible unless we sup- 

 pose a double refraction. For a ray of the sun coming from a 

 point in the sun through the centre • of the flask is not refracted, 

 because it falls perpendicularly on flask, water, and air, passing 

 through the centre of each (Fig. 3). . . . But all the (other) 

 rays which are given forth at the same point in the sun from 

 which this perpendicular ray comes are necessarily refracted on 

 the body of the flask, because they fall at oblique angles, and since 

 the flask is denser than air, the refraction passes between the straight 

 path and the perpendicular drawn from the point of refraction 

 to the centre of the flask. And when it passes out again into the 

 air, then, since it comes upon a less dense body, the straight path 

 passes between the refraction and the perpendicular drawn from 

 the point of refraction, so that the refracted ray may fall upon 

 the first perpendicular which comes without refraction from the sun. 

 And suice an infinite number of ^^ays are given off from the same 



