ANALYTIC CRYSTALS. 



40 ] 



ANAULUS. 



uric acid, Sec. They must be immersed in 

 Canada balsam. The cryi^talline compound 

 of disidphate of quinine with iodine is infe- 

 rior to none in this power. The phenomena 

 scarcely need description, since analytic crys- 

 tals merely play the part of a thick plate of 

 tourmaline, or a Nicol's prism : i. e. if polar- 

 ized light be transmitted through them (a 

 polarizer alone being used), in one position 

 they sutler it to pass freely, while if they are 

 rotated 90° they arrest or absorb it entirely, 

 or to a greater or less extent : and if a plate 

 of selenite, or other depolarizer, be placed 

 beneath the slide upon which the crystals 

 are situated (without the analyzer) the lat- 

 eral surfaces are seen to be coloured, the 

 complementary tints appearing at each 

 quarter rotation. 



Of course these crystals will act equally as 

 polarizers and analyzers. Fox Talbot gives 

 the following explanation of the cause of the 

 phenomena in the crystals which he exa- 

 mined. When a beam of polarized light is 

 transmitted very obliquely through a small 

 prism of nitre, its outUne generally exhibits 

 two colours instead of one ; for while the 

 edge of the prism, which is on that side 

 from which the ray of light comes, is, for 

 instance, red, the opposite edge will appear 

 green. On reversing the polarization of the 

 light, these colours are exchanged. This ob- 

 servation shows why the phenomenon only 

 occm'S in crystals possessing strong double 

 refraction, like nitre, in which the refractive 

 indices of the two rays are materially dif- 

 ferent. When a ray of common light is in- 

 cident upon such a crystal, and therefore is 

 divided into two rays oppositely polarized, 

 both rays are transmitted through the cen- 

 tral parts of the crystal, which are bounded 

 by parallel planes, or by planes approaching 

 to parallelism. But when tlie bounding planes 

 of the crystal are much inclined to each other, 

 and therefore refract the light in the manner 

 of a prism, the refi'active indices of the rays 

 mav diflPer so much, that while one passes free- 

 ly through such a prism, theothercannot pass 

 at all, but suffers total internal reflection, 

 and is thereby dispersed ; just as if the prism 

 had a larger "^reflecting angle with respect 

 to that ray than to the other. Therefore if 

 two op])ositely polarized rays are presen- 

 ted to such a crystal as in our experiment, 

 one will be transmitted and the other not. 

 That this is the true explanation appears 

 from this, that when the obhque planes are 

 well-formed and clearly defined by the mi- 

 croscope, the colour also is accurately limited 



by the same boundary ; so that while this 

 part analyzes the tints of a plate of sulphate 

 of lime, the rest of the crystal is inactive. 



That internal reflection and dispersion, 

 however, are not the cause of the separation 

 of the coloured rays, is shown by the fact 

 that those lateral surfaces of crystals which, 

 when viewed through the microscope (with 

 the polarizer and plate of selenite alone ), ap- 

 pear of a certain colour, say green, exhibit 

 the complementary tint, red, when viewed 

 with the naked eye from the side of the 

 stage ; hence the two coloured rays are se- 

 parated merely by refraction. 



The margins of cavities containing air and 

 air-bubbles, which sometimes exist in the 

 crystals, exhibit the colours in the same 

 manner and from the same cause as the 

 lateral oblique surfaces of the crystals. 



Nothing can sui'pass the curious and beau- 

 tiful appearance presented by analytic crys- 

 tals, the delicacy and briUiant transparency 

 of their coloured margins giving them the 

 aspect of figures drawn ^"ith coloured ink. 



PL IL fig. 11 a, ft, represent two crystals of 

 nitre, viewed with the polarizer but with (.)ut 

 the analyzer or the plate of selenite; fig. 12 

 a, 6, represent two crystals as seen when the 

 polarizer and jilate of selenite are used, exhi- 

 biting the complementary colours; fig. 12 c 

 represents an air-bubble enclosed in the crys- 

 tal. See DiCHROiSM and Polarization. 



BiBL. Brewster, P/nl. Trans. 18.3.5 ; Fox 

 Talbot, ibid. 18.37. 



ANAliTIIROP'ORA, ^m\ti,= Lejmilia 

 part. 



A. monodon=Lepr. man. ; on stones from 

 deep-sea water. 



BrBL. Hincks, Pohjzoa, p. 232. 



ANAU'LUS, Ehr.— A genus of Diato- 

 maceae. 



Char. Frustules single, compressed, sub- 

 quadrate, not furnished with either tubular 

 processes, nodules or aperturesi, but ha\ ing 

 lateral constrictions. 



In the latter character it resembles Bid- 

 dulphia. 



Kiitzing admits one species : — 



A. scalaris, Ehr. (PI. 18. fig. 7). Valves 

 turgid in the young state, very broad and 

 flat when mature; having 4, G, 8, or 14 

 lateral constrictions, which give the front 

 view a ladder-like appearance; marine; dia- 

 meter 1-470 to 175". Antarctic Ocean. 



A. indicus, Ehr.= Terpsinoe indica, Kiitz. 



BiiJL. Ehrenberg, Ber. d. Berl.Ak.l8U, 

 p. 197; 1845, p. 3l3l; Kiitzing-, -S^^. Ala. pp 

 119, 120. 



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