19. Such accidents never happen to common men. Thousands of men, even of the most inquiring and speculative men, had seen bodies fall; but who, except Newton, ever followed the accident to such consequences? And in fact, how little of his train of thought was contained in, or even directly suggested by, the fall of the apple! If the apple fall, said the discoverer, 'why should not the moon, the planets, the satellites, fall?' But how much previous thought,--what a steady conception of the universality of the laws of motion gathered from other sources,--were requisite, that the inquirer should see any connexion in these cases! Was it by accident that he saw in the apple an image of the moon, and of every body in the solar system? 20. The same observations may be made with regard to the other cases which are sometimes adduced as examples of accidental discovery. It has been said, 'By the accidental placing of a rhomb of calcareous spar upon a book or line Bartholinus discovered the property of the _Double Refraction_ of light.' But Bartholinus could have seen no such consequence in the accident if he had not previously had a clear conception of _single refraction_. A lady, in describing an optical experiment which had been shown her, said of her teacher, 'He told me to _increase and diminish_ {48} _the angle of refraction_, and at last I found that he only meant me to move my head up and down.' At any rate, till the lady had acquired the notions which the technical terms convey, she could not have made Bartholinus's discovery by means of his accident. 'By accidentally combining two rhombs in different positions,' it is added, 'Huyghens discovered the _Polarization_ of Light.' Supposing that this experiment had been made without design, what Huyghens really observed was, that the images appeared and disappeared alternately as he turned one of the rhombs round. But was it an easy or an obvious business to analyze this curious alternation into the circumstances of the rays of light having _sides_, as Newton expressed it, and into the additional hypotheses which are implied in the term 'polarization'? Those will be able to answer this question, who have found how far from easy it is to understand clearly what is meant by 'polarization' in this case, now that the property is fully established. Huyghens's success depended on his clearness of thought, for this enabled him to perform the intellectual analysis, which never would have occurred to most men, however often they had 'accidentally combined two rhombs in different positions.' 'By accidentally looking through a prism of the same substance, and turning it round, Malus discovered the polarization of light by reflection.' Malus saw that, in some positions of the prism, the light reflected from the windows of the Louvre thus seen through the prism, became dim. A common man would have supposed this dimness the result of accident; but Malus's mind was differently constituted and disciplined.