a»6 Onihe unufualHourfe ef Light through Iceland Cryftal. 



as follows : When the cryftal is turned round, the unufual image turjis round alfo, and ap- 

 pears above the other : the grcateft diftance between the two images is when they are 

 parallel to the line bifefling one of the acute angles of the parallelogram through which the 

 rays pafs : when the images are parallel to a line bifedling one of the obtufe angles they 

 feem to coincide; but they will be found, if obferved more nearly, to coincide only in part. 

 "Thus (in fig. 9, plate Vll.) A B and CD are the two black lines at their greater diftance, and 

 their extremities A and C, B andD, are even with one another; that is, the figure formed by 

 ■joining A and C, B and D, is a reSangle. But in the other cafe (fig. 8), A B and C D being 

 the lines, the fpace CB (equal in deptli of colour to the real line on the paper) is the only 

 j)lace in which the lines (or images) coincide. The fpace A C of A B, and B D of CD 

 are ftill of alight colour, and the two lines AB and CD do not coincide, by the difference 

 A C or B D i that is, by the difi^srence O P, the greateft diftance (fig. 9). In (hort, the un- 

 ■ufualline's extremities defcribe circles (in the motion of the cryftal) whofe centres are the 

 extremities of the ufual line, and whofe radii are the greateft diftance. From this it appears 

 ■evident, that the unufual image is formed within the cryftal, and turns round with the fide 

 •of the particle or rhomboidal mafs of particles which forms it. Farther, it is evident that 

 -the power which produces the divifion of the incident light is very different from common 

 refraftion, from the motion, and the £ffe£t taking place when the rays are perpendicular. 

 Sufpeding, therefore, that it might be owing to flexion, I made the following experiment, 

 which undeceived me : 



Obfervation 5. I covered one fide of a fpecimen of Iceland cryftal three inches deep witlt 

 ■tlack paper, all but a fmall fpace 3-0 of an inch in diameter, and placed a fcreen with a 

 hole of the fame fize, fix feet from the hole in the vvindow-ftiut of my darkened chamber, fo 

 that the rays might pafs through the fcreen and fall on a prifni placed behind, to refracSt 

 them into a fmall and well defined fpe£trum, which was received on a chart two feet from 

 the prifm. This fpedrum I viewed through the cryftal, and of courfe faw it doubled ; but 

 the two images were by no means parallel : the unufual one inclined to the red, and its violet 

 was confiderably farther removed from the violet of the other, than the two reds were from 

 one another ; which fhews that the moft refrangible or leaft flexible rays were fartheft moved 

 from their courfe by the unufual action, and proves this to be very diiFerent from 

 flexion*. 



From all thefe obfervations this conclufion follows : that the remarkable phenomenon in 

 queftion arifes from an adlion very different from either refraftion or flexion, and whofe na- 

 ture well deferves to be farther confidered. It may poffibly belong to the particles of Iceland 

 cryftal, and in a degree to thofe of rock cryftal, from the form and angles of the rhomboidal 

 maffes whereof thefe bodies are compofed. Nor is this conjefture at all difproved by the fail, 

 that glafs fliaped like thefe bodies wants the property ; for we cannot mould the particles of 

 glafs, we can only fliape large malTes of thefe ; whereas we cannot doubt that in cryftallization 

 the fmalleft mafTes afTume the fame form with the largeft. But then other hypothefes may 

 perhaps alfo account for the fail, fuch as atmofpheres, eletSlric fluid, &c. &c. ; fo that till 

 farther obfervations are made we ought to reft contented with barely fuggefting the query. 

 ,Xn the mean time, referving to a future opportunity fomc inquiries concerning the chemical 



• When a candle cr line is viewed tlirough a deep fpecimen, the unufual image it tinged with cotours. 



propertJM 



