OF NEWTON'S OPTICS. 



frangibility of light. Here, as in the 

 case of the law of refraction, this geo- 

 meter has fixed an indelible stain upon 

 his fame, by a similar act of unprinci- 

 pled selfishness and injustice towards 

 Antonio de Dominis, to whom he never 

 even alludes in his treatise. Descartes 

 explained why the interior bow has a 

 diameter of 42 ; this was an easy in- 

 ference from the theory of De Dominis 

 and the law of Snellius. But the genius 

 of Descartes was very conspicuous in 

 his solution of the problem of the ex- 

 terior bow. He shewed that the sun's 

 rays entering the inferior part of the 

 drop, emerged with a second refraction 

 from the superior, after undergoing two 

 reflections within. The double reflec- 

 tion accounted for the faintness of the 

 exterior bow, and the inversion of its 

 colours. Thus we see that ignorance of 

 the different refrangibility of light alone 

 prevented this philosopher from bringing 

 the theory of the rainbow to absolute 

 perfection. 



(13.) In the middle of the seventeenth 

 century a discovery was made by Eras- 

 mus Bartolinus, a Danish mathemati- 

 cian, which formed the first of the most 

 brilliant train of experiments, and may 

 be considered the basis of the most 

 splendid speculations which ever adorn- 

 ed the annals of science ; speculations 

 which regard not merely the phenomena 

 of light and vision, but which seem to 

 furnish man with new sensibilities, which 

 are to touch what the microscope is to 

 sight, and disclose to his view wonders 

 of nature which would refuse to un- 

 veil themselves to any other power. In 

 this extensive field, more than in any 

 other, has the philosophic genius of 

 our own times shone forth. 



Bartolinus received from some Danish 

 merchants, who frequented Iceland, spe- 

 cimens of the crystal, of extraordinary 

 transparency and dimensions, since 

 known by the name of Iceland spar, 

 which is carbonate of lime, in a crystal- 

 lized form. While making optical ex- 

 periments with pieces of this crystal, he 

 discovered that it exhibited a double 

 image of objects seen through it, and 

 consequently inferred that each ray of 

 light in passing through it was cloven 

 into two rays, which were refracted at 

 different angles, while the angle of in- 

 cidence of the whole ray of which they 

 were the component parts remained the 

 same. Slight observation was sufficient 

 to prove that both of these refracted 

 rays did not obey the Snellian law, for 



if that were the case, both images of an 

 object seen through the crystal would 

 maintain the same position with respect 

 to a perpendicular to the refracting sur- 

 face, while the crystal was turned round 

 that perpendicular as an axis. Such, 

 however, he found not to be the fact. 

 By the revolution of the crystal, one of 

 the images only was observed to obey 

 the above-mentioned law. Hence he 

 inferred that one of the two refractions 

 was performed according to the com- 

 mon law, but that the other obeyed an 

 extraordinary law, not before noticed by 

 opticians. The results of these experi- 

 ments were published by Bartolinus, in 

 a work, entitled Experimenta crystalli 

 Islandici dis-diaclastici quibus rnira et 

 insolita ref radio detegitur. Copen- 

 hagen, 1699. 



The publication of this work soon 

 drew the attention of the celebrated Huy- 

 gens to the subject of double refraction. 

 Previously to this he had published a 

 theory of refraction and reflection, found- 

 ed on the hypothesis, that light, like 

 sound, was propagated by the undula- 

 tions of a subtle and elastic medium, 

 which he supposed to pervade all 

 space permeable to light. Others held 

 that light was corporeal, and com- 

 posed of infinitely small and subtle 

 corpuscles, which were emitted from 

 every luminous body, and entering 

 the eye impinged upon the retina, and 

 produced sensation. This corpuscular 

 theory, which is as old as Pythagoras, 

 was adopted by Newton for the expli- 

 cation of the phenomena of optics ; but 

 it is proper to add, that he is careful not 

 to intermingle with the reasoning from 

 his experiments any assumptions from 

 this theory, which could at all affect the 

 validity of his results, all of which are 

 entirely independent of any hypothesis, 

 and such that any theory must account 

 for before it can be admitted as a true 

 one. 



Huygens took up the subject of double 

 refraction, in order to obviate any ob- 

 jections to his undulatory theory, which 

 might arise from it. He accordingly 

 explains distinctly the law which regu- 

 lates the refraction of the extraordinary 

 ray, and attempts to shew how the phe- 

 nomenon may be accounted for by sphe- 

 roidal undulations, while ordinary re- 

 fraction is ascribed to spherical undu- 

 lations. As, however, this part of the 

 subject is not intimately connected with 

 the discoveries of Newton, we shall not 

 here enter into further detail upon it, 



