t 



ARE 



«fior,fd » rrtich p-catrr A ini'-Uio" '^- tlic i-xtienic r.ws thun 

 the flint glafs wUwh Mr. l).>ll..iiJ Imtl maJc iitc of, ;iiul at 

 tljc fanic time C')r.fi<lir.ibly inorciiftd the mean lefr.idioii. 

 M. Ztihtr, in the cuirft of his experiments, made £(l:ilt 

 of minium arA fimt, with a mixture pf alkahne f;dts ; and 

 found that this mixture jjri'Jtly dimiiiilhed the mean refrac- 

 tion, without rrakinj,' hardly any chnnpe in the difperiion ; 

 •nd he at length obtaiieJ a kir.d'of glals greatly fupenor to 

 the flint glafs of Mr. DoIKwd for the conftniction of tekf- 

 copci; fir.ce it orcafioncd itrr:- times as great a difperiion of 

 the rays as the common glafs, whilll the mean refraction 

 was only a-. 1.61 to i. 



Other kinds of metallic glafs, as well as that of lead, pof- 

 fefs the fame uftfnl property of difperiion. Some philofo- 

 phcrs have, indeed, imagined, that there is a conftant rela- 

 tion between the fpecific gravity of glafs and its dilperfion; 

 but it ought to be confidercd, that the difperiion in a;thcr 

 «nd fpirit of wine is ilrongcr than that produced in water, 

 vrhich i; a much heavier lluiJ. The object which opticians 

 have generally propofed by ufing flint and common glafs in 

 the conftniftion of achromatic telefcopes, might be attained 

 by means of other tranfparent fubilances, both folid and 

 fliiid. Dr. Blair of Edinburgh has lately evinced this fact. 

 riint glafs was found to reflect the green light confiderably 

 Kf« than crown glafs, in proportion to the whole refra<;T;ion 

 of red and violet light ; fo that when the divergency of the 

 red and that of the violet light, caufed by the refraftion of 

 the two mediums, arc equal, the divergency of the red and 

 green light is always greater in the crown than in the flint 

 jlafs, and the divergency of the violet and green is always 

 lefs in the crown than in the flint glafs. After a variety of 

 experiments. Dr. Blair difcovercd that the muriatic and ni- 

 tric acids, which are difperfive fluids of confiderable ftrength, 

 inftead of refrafting the green hght lefs than crown glafs, 

 in proportion to the whole refraSion of the red and violet 

 light, refrafted the green light more than crown glafs, in 

 proportion to the whole refraction of red and violet light. 

 He, therefore, mixed thefe two kinds of difperiive mediums, 

 and thus obtained a medium, which difperfes the rays much 

 more than crown glafs, and yet caufes them all to diverge 

 accurately in the fame proportion in which they are made 

 to diverge by the refraftion of crown glais; which entirely re- 

 moves the aberrolion from the unequal refrangibility of light. 



In 1758, Mr. DoUond made a farther improvement in 

 telefcopes, by introducing two object-glaffes of crown glafs, 

 and one of flint. For various papers on the fubject of this 

 article, fee Phil. Tranf. vols. 35. 48. 50.51. 52. 55.60. 

 Hilt. Ac. Par. for 1 7 37 — 46 — 52 — 55 — 56 — 57 — 62 — 64 — 

 65 — 67 — 70. Swed. Mem. vol. xvi. Ac. Berl. 1746, 1762, 

 and 1766. Com. Nov. Petrop. 17C2. Euler's Dioptrics. 

 M. D'Alembert's Opuf. Mathcm. Edinburgh Tranfattions, 

 vol. iii. pt. 2. p. I. — 76. 



For a farther account of Mr. Dollond's and other new re- 

 fracting telefcopes of the achromatic or aplanatic kind, fee 



TlLESCOPE. 



Aberration, croiun of, is a luminous circle fuiTounding 

 the real di(k of the fun, and depending on the aberration of 

 the folar rays, whereby his apparent diameter is enlarged. 



ABERSPERG, in Geography, anciently Jlliufma Or 

 jivcnt'mum, a fmall town and caflle in Upper Bavaria, feated 

 on the river Umbs, and containing a convent of Carmehtes. 

 It is famous as the birth-place of Johannes Aventinus. 



ABERYSTWITH, in Geography, a market town of 

 Cardiganrtiire in Wales, on the Rhydol, near its confluence 

 with the YlV.vith, where it falls into the foa. It is fituate 

 on a bold eminence overhanging the fea, or bay of Cardigan. 

 The ftreets are fleep and ill-paved, and the houfes are co- 

 »ered with the black Hate of the countr)-. It canies on a 

 1 



A B p: 



trade in lead, Cdl.ir.ii.sc, and Jifli, parliciilurly herrings, 

 v.'hich lalt brancli has lately declined ; and a few manu- 

 faiftared goods, f'.icli as webs, fiannd'?, and (l( -kings. Of 

 late it is lias been improving, and is become a place of refort 

 for fea-hathiug. Its walls and cadle arc in ruins. The calllc 

 was built by (Gilbert de Strongbow, in the reign of Henry I. 

 and commands the whole coail with the contiguous mouths 

 of th.' Yil'.vitli and Rhydol on one fide, and a beautiful view 

 of the vale which defcends with t'le river on tiie other. A 

 mint for the coinage of filvcr was ellablilhed in this place by 

 king Charles in 1637, and the coin was to be llamped on 

 both lldes v.ith the Feathers, in order to Ihew that it was 

 coined in Wales. It is 203 miks W. N. W. of London. 

 ^V. long. 4°. N. lat. 52"^ 5'. 



ABERMENAI, a pad'a^e at the S. W. end of the Me- 

 nai llraits, re]).)rating Carnarvonfliire from Anglefea.. 



ABERV1LLE'/-;V/-, abranch of the Millilippi, in New 

 Orleans, communicating with the lakes which fall into St. 

 Louis bay. 



ABESTA, or AvESTA, the name of one of the facred 

 books of the Perfian Magi ; which they attribute to their 

 great founder Zoroaller, or Zerduflit. 



The Ahtjla is a commentary- or expofition of two other of 

 their religious books, called Zend and Pazend ; the three 

 together include the whole fyilem of the Ignicole, or wor- 

 fliippers of fire. D'Herbel. Bibl. Orient, p. 11. Hyde de 

 Rcl. vet. Perf. c. 2. 



According to Du Perron, the A'oejla fignifies the lan- 

 guage of the Oriental text of Zoroafter's works. Hift. Ac. 

 Sc. Par. 1762. See Shanscr.it and Shastah. 



ABESTON. See Asbestos. 



ABETHANCOURT, Jacques, \n Biography, aphyfi- 

 cian at Rouen, puUifhed Nova penitentialis quadrigefima, 

 et Purgatorium in Morbum Gallicum, five venereum, una 

 cum Dialogo Aqux Argenti, ac Ligni Guaiaci colluftantium 

 fuper diCti Morbi Curationis prelatura. Opus fruftiferum, 

 Parilils, 1527, in 8vo. 



He is the firft; French phyfician \\'ho wrote on the vene- 

 real difeafe. Perhaps, M. Eloy fays, becauie the difeafe 

 appeared at Rouen, before it was communicated to other 

 parts of the kingdom. 



ABETTOR, or Abbettor, in Law, one who incites 

 or encourages another to perform fomething criminal; or by 

 fome way afiifts him in the performance itfelf. It is the 

 fame with art and part in the Scots law. 



Thus thofe who procure others to fue out falfe appeals of 

 felony, or murder, againft men, to render them infamous, 

 are particularly denominated abettors. 



So abettors in murder, are fuch as advife or procure a 

 murder to be committed, or are acceflary thereto. 



There are abettors in felony, but not in treason ; 

 the law looking on r.U thofe concerned in treafon as princi- 

 pals. 



ABEVACUATION, in Medk'we, denotes a partial 

 evacuation of the peccant humours, either by nature or art. 



ABEX, or Abesh, in Geography, a country of Ethiopia 

 in Africa, bordering on the Red Sea, which bounds it on the 

 eaft. It has Nubia on the north, Abyffinia on the weft and 

 fouth. Its principal towns are Suaquem and Arkeko. It 

 has the name of Beglerbeg of Habeleth, and is fubjedt to 

 the Turks. It is a fandy, barren, unhealthy country, about 

 500 miles in length, and roo in breadth. Being very moun- 

 tainous, it abounds with wild beafts; and in its forells there 

 are many ebony-trees. See Anian. 



ABEYANCE, Abeiance, or Abbayance, in Law' 

 boots, fomething that only exifts in expetlation, or in the 

 intendment, or remembrance of the law. 



/ibeyance, in our law, amounts to much the fame with A*- 



redito* 



