S E L 



S E R 



optic axes to the vifual bafe, do not appear to be in that 

 line, but in another, drawn through the fame interfeftion, 

 to a point in the vifual bafe diftant half this bafe from the 

 fimilar extremity of the former line, towards the left, if the 

 objetls be feen by the right eye, but towards the right, if 

 feen bv the left eye. In the application of the theory 

 which Dr. Wells has endeavoured to ellablifh in the pre- 

 ceding propofitions, he obferves, that, if the queftion be 

 concerning an objeft at the concourfe of the optic axes, it is 

 feen lingle, becaufe its two fimilar appearances, in regard 

 to fize, lliape, and colour, are feen by both eyes in one and 

 the fame direftion, or, if you will, in two directions, which 

 coincide with each other through the whole of their extent. 

 It therefore matters not, whether the diftance be truly or 

 falfely eftimated ; whether the objeft be thought to touch 

 our eyes, or to be infinitely remote. And hence we have a 

 reafon, which no other theory of vifible direftion affords, 

 why objefts appeared fingle to the young gentleman men- 

 tioned by Mr. Chefelden, immediately after his being 

 couched, and before he could have learned to judge of 

 diftance by fight. 



When two fimilar objefts are placed in the optic axes, 

 one in each, at equal diftances from the eyes, they will 

 appear in the fame place, and therefore one, for the fame 

 reafon that a truly fingle objeft, in the concourfe of the 

 optic axes, is feen fingle. Here again, as the two vifible 

 direftions coincide in every point, it is not necefiary that 

 the united appearance (hould be judged to be at any par- 

 ticular diftance ; that it ftiould be referred, for initance, 

 to the concourfe of the optic axes, where the two other 

 theories of vifible direftion are obliged to place it, in oppo- 

 fition to the plaineft obfervation. 



Objefts, any where in the horopter, will be feen fingle, 

 becaufe their apparent direftions to the two eyes will then 

 completely coincide. And for a contrary reafon, thofe 

 placed in any other part of the plane of the optic axes will 

 appear double. To make thefe things evident, let a line 

 pafs through the point of interfeftion of the optic axes and 

 any given objeft, to the vifual bafe, which is to be produced, 

 if neceflary ; and let it be called the line of the objeft's real 

 pofition. Take afterward, in the vifual bafe, or its pro- 

 duftion, two points, one on each fide of the line of real 

 pofition, and both diftant from its termination there, half 

 the vifual bafe. Lines drawn from thefe points, through 

 the point of interfeftion of the optic axes, muft confequently 

 contain the two vifible pofitions of the objeft. But when 

 this is fituated in the horopter, the line of real pofition will 

 coincide with the horopter, and will not therefore reach the 

 vifual bafe, unlefs at an infinite diftance from the eyes. For 

 which reafon, the two lines, containing the vifible pofitions 

 of the objeft, muft fall upon the vifual bafe at a like dif- 

 tance, and muft confequently be regarded as coinciding 

 with each other. When the objeft is not in the horopter, 

 the two lines of vifible direftion will be found, by the 

 fame means, not to coincide. 



SEGO, col. 2, 1. 13, r. Manfong ; 1. 17, r. San- 

 fanding. 



SELENIUM, in Chetni/lry, the name of an elementary 

 fubftance recently difcovered by Berzelius, and confidered 

 by him as a kind of femi-metal. This fubftance was firft 

 miftaken for tellurium. It was obtained from a ful- 

 phuric acid manufaftory at Gripfholm, where pyrites from 

 the mines of Fahlun were employed, and which of courfe 

 contained the fubftance in queftion. It alfo exifts in the 

 fame mines combined with copper. Selenium has the pro- 

 perties of a metal combined with thofe of fulphur to fo 

 great a degree, that it is difficult to know under which 



Vol. XXXIX. 



head to clafs it, and in ftiort whether it might not be 

 rather confidered as a new fpecies of fulphur. In its me- 

 tallic ftate it has a brilliant metallic luftre externally, with a 

 tinge of red. The frafture is vitreous, like that of fulphiu-, 

 but with a very brilliant luftre of a grey colour. It be- 

 comes foft at a temperature of 2 1 2°, and at a higher tem- 

 perature it melts ; and at a temperature about equal to that 

 at which mercury boils it may be diftilled. When in a 

 gafeous ftate, it is yellow, like fulphur. When fublimed 

 in a large veffel, it is depofited in the form of flowers of a 

 cinnabar colour, but not oxydized. During its cooling, it 

 preferves for fome time a certain degree of fluidity, fo that 

 it may be moulded between the fingers, and drawn into 

 threads, which are tranfparent, and of a ruby-red colour 

 when held between the eye and the light ; but by reflefted 

 light they exhibit a brilliant metaUic luftre. It burns 

 with an azure -blue flame when heated with a candle, and 

 exhales a ttrong odour of horfe-radifli. 



Selenium combines with metals, and generally produces 

 a reddifti flame. The alloys are commonly grey, with a 

 metallic luftre. The feleniuret of potaflium diftolves in 

 water, without evolving any gas, and produces a red- 

 coloured folution, which has the tafte of hydrofulphuret of 

 potafli. When muriatic acid is poured upon this feleniuret, 

 a feleniuretted hydrogen gas is difengaged, which is foluble 

 in water, and precipitates all metallic folutions, even thofe 

 of zinc and iron. This gas has the odour of fulphuretted 

 hydrogen gas when it is diluted with air, but if it be 

 breathed lefs diluted it produces a painful fenfation in the 

 nofe, and a violent inflammation, ending in catarrh, which 

 continues for a confiderable time. 



Selenium combines with the alkalies both in the humid 

 way and by fufion. Thefe combinations are red. The 

 feleniurets of barytes and lime are alfo red, but they are 

 infoluble. It alfo diflblves in melted wax, and in the fat 

 oils ; the folutions are red, but have no hepatic odour. 

 There exift alfo feleniuretted hydroleleniurets of the alkalies 

 and of the earths. 



Selenium diflblves in nitric acid by the afliftance of heat ; 

 the folution evaporated and fublimed yields a mafs cryf- 

 tallized in needles, which is a pretty ftrong acid. It has a 

 pure acid flavour, and forms peculiar falls with the alkalies, 

 earths, and metallic oxyds. The felenic acid is foluble in 

 water and in alcohol : its combinations with potafli and 

 ammonia are deliquefcent ; the latter is decompofed by fire, 

 water is given out, and the felenium is reduced The 

 feleniates of barytes and lime are foluble iu water. The 

 felenic acid mixed with muriatic acid is decompofed by 

 zinc, and the felenium is precipitated in the form of a red 

 powder ; by fulphuretted hydrogen gas an orange-yellow 

 precipitate is formed. 



Such is a brief fummary of the properties of this curious 

 fubftance. From the fmall quantities in which it has 

 hitherto been found, we beheve no experiments have been 

 made to afcertain the weight of its atom, &c. 



SELKIRK, 1. 23 from bottom, for 440 r. 439. 



SELKIRKSHIRE. In 181 1 this fliire contained 

 1080 houfes, and 5889 perfons ; viz. 2750 males, and 

 3139 females: 500 families being employed in agriculture, 

 and 363 in trade, manufaftures, and handicraft. 



SENECA. Add— Alfo, a county of New York, con- 

 taining 16,609 inhabitants, of whom loi ai-e flaves. — Alfo, 

 a town of Guernfey county, in Ohio, having 300 inhabitants. 



SEPOY, a term ufed in India to denote a native foldier. 



SERA-WOLLIES. See Kajaaga. 



SERICA. Add— (See Thibet.) Hugh Murray, efq. 



in his " Ancient Geography of Eaftern and Central Afia," 



4 N publiftied 



