114 DIFFRACTION DOUBLE EFFRACTION. 



And for the bow by two reflections : 



Violet rays . r,. = 71° 49' 55". J"y = 230^ 58' 50. Radiiis of bow 50° 58' 50". 

 Ucdrny«.--;,-=7i°26'10". J"i. = 23'r 9' 20. Eadius of bow54° 9' 20". 



From the values of J it will be manifest that the rays which produce the 

 bow by one reflection must enter the rain drops above the ray which passes 

 throu"-h the centre; and that those which produce the bow by two reflections 

 must enter below the same central ray. 



The differences between the values of J^- and A^. above, show the amount by 

 v,'hi<.-h tlu' breadths of the bows are increased in consequence of the variabilily 

 of n. These amounts arc, for the lirst bow, 1° 44' 40'', and for the second, 

 o^ 10' 30 '. The colors are produced by the want of conformity of the bows 

 corresponding to the several elementary rays; and their feebleness is owing to 

 the fact that, notwithstanding this want of conformity, they do, on account of 

 the considerable diameter of the sun, very sensibly overlap, while they are 

 also diluted by the white light reflected from the anterior surface of the drops 

 Were they entirely superposed upon each other the bow would be white. 



While the discoveries of Newton and Snellius, just mentioned, were removing 

 old impediments to progress in optical science, observation continued to add 

 new ones more perplexing than those which had disappeared. In the year 

 1G65 there was published, at Bologna, a posthumous work by Francis Maria 

 Grimaldi, an Italian Jesuit, in which were, for the first time, described certain 

 phenomena now very familiar under the name of diffraction. He stated that 

 if any very small object be placed in a pencil of divergent light, admitted through 

 a mmute aperture into a dark room, its shadow will appear materially larger 

 than it ought if light passes its edges in straight lines ; and. moreover, that any 

 opaque object, laige or small, exhibits along the edges of its shadow a border 

 of at least three distinctly tinted fringe!*, the brightest and broadest of which is 

 next the shadow. He also observed that when two minute pencils of light are 

 admitted through apertures very near to each other, the screen on which the 

 blended pencils fall, and which, as he supposed, ought to be uniformly UIu- 

 mmated with a light equal to the sum of the two intensities, is streaked with lines 

 absolutely dark. He was led by this observation to announce the paradoxical 

 proposition that there are circumstances in which the union of two rays of light 

 produces darkness. Bold as this announcement must have originally appeared, 

 the progress of scientific discovery has fully confix-med its truth. This phenom- 

 enon, bemg attributed to the bending of the rays of light in the immediate 

 vicinity of the opaque body, was distinguished by the name inflection or diffrac- 

 tion. It was carefully studied by Newton and others, and has occupied a prom- 

 inent place in all the discussions which have since arisen in regard to the natui-e 

 of light. 



Noi far from the time of the discovery of Grimaldi, just mentioned, the atten- 

 tion of the; scientific world was called to a case of new and extraordinary r(>irac- 

 tiou observed to take place in crystals of carbonate of lime — a species of retrac- 

 tion, which, from tiie circumstance of its dividing an meideut beam into two 

 beams entirely distinct, or of presenting two im.iges of any object seen through the 

 crystal, has been catted double refraction. 1 he first publication on this subject 

 was made by Erasmus Banholinus, a physician of Copenhagen, who gave to 

 the mineral the name of Iceland spar, from the circumstance that his specimens 

 had been obtained from that island. It is now known that this property of 

 double refraction is exceedingly common, being possessed by most crystaUized 

 bodies, and capable of being produced, transiently or permanently, in any trans- 

 parent solid whatever, whether organic or mineral, in which it does not naturally 

 exist. It is only in Iceland spar, however, that it manifests itself in a degree 

 remarkable enough to attract; the attention of a casual observer, and in most 

 cases It can oulj be detected by special arrangi^neuts. 



