S P E 



is a double concave glafs, ground on a tool of 8 inches 

 radius on one fide, and ii inches on the other, the mean 

 between which is very nearly 9 inches. From the whole 

 ftatement of our author, he infers, i. That near-fightednefs 

 it rarely obferved in infants, or even in children under ten 

 years of age. It affefts the higher clafles of fociety more 

 than the lower ; and few inftances occur, if any, in which, 

 if the ufe of concave glaifes has been adopted, increafmg 

 years have either removed or leflened this imperfeftion. 

 2. Although the ufual effeft of time on perfeft eyes is that 

 of inducing a neceflity to make ufe of concave glades, in 

 order to fee near objefts diltinftly, yet foraetimes, even after 

 the age of 50, and after convex glaffes have been ufed many 

 years for this purpofe, the eyes have not only ceafed to de- 

 rive benefit from them, when looking at near objefts, but 

 they have required concave glafies to enable them to diftin- 

 guifh, with precifion, objefts at a diftance. 3. Although 

 the caufe of this change be not always known, yet fome- 

 times it has been induced by the ufe of evacuating remedies, 

 particularly of leeches apphed to the temples ; and forae- 

 times by looking through a microfcope, for a confiderable 

 length of time, on feveral fuccefTive days. 4. Initances are 

 not uncommon, in which perfons far advanced in life, (vix. 

 between 80 and 90,) whofe eyes have been accuftomed for 

 a long time to the ufe of deeply convex glaflcs, when 

 they have read or written, have ceafed to derive benefit from 

 thefe glades, and they have become able, without any af- 

 fiftance, to fee both near and diftant objefls almoil as well as 

 when they were young. This change is probably owing to 

 an abforption of part of the vitreous humour, by which the 

 axis is lengthened ; and thus its length, from the cornea to 

 the retina, is brought into the fame proportion to the flat- 

 tened ftate of the cornea, or cryftalline, or both, which it 

 had to thefe parts before the alteration took place. 



SPECTATOR, a perfon prefent at a fpeftacle. 



Among the Romans, fpeftators, fpeftatores, more par- 

 ticularly denoted a kind of gladiators, who had received 

 their difcharge ; and were frequently hired to be prefent as 

 fpeftators at the combats of gladiators, &c. with which the 

 people were entertained. See Rudiarius. 



SPECTER, in Conchohgy, a name given by the French 

 naturalilts to a fpecies of voluta, on which there are feveral 

 reddifh broad bands, compofed of loofe and irregular figures ; 

 the ground colour is a fine white. They are called by Lutin 

 writers concha fpeSrorum. 



SPECTRORUM Candela, in Natural Hiftory, a name 

 by which fome have called the belemnites. 



SPECTRUM, Ocular, in Optics, a name given by Dr. 

 Darwin (Phil. Tranf. for 1786) to an image, refembling 

 in f )rm the objetl to which the obferver was attending, and 

 which continues for fome time to be vifible, on clofing his 

 eyes or covering them, after having been long and atten- 

 tively looking at a bright objeft, fuch, c. g. as the fetting 

 fun. Thefe ocular fpeftra, he fay?, are of four kinds"; 

 i»;s. I. Such as are owing to a lefs fenfibility of a defined 

 part of the retina, or " fpeftra from defeft of fenfibility." 



2. Such as are owing to a greater fenfibihty of a defined 

 part of the retina, or " fpeftra from excefs of fenfibility." 



3. Such as refemble their objed in colour as well as in form, 

 which may be termed " direft ocular fpeftra." 4. Such as 

 are of a colour contrary to that of their objeft, which may 

 be termed " reverfe ocular fpeftra." The author, from ex- 

 periments which he has detailed, concludes, that the retina is 

 in an aftive, not in a paffive ftate, during the exiftence of 

 thefe ocular fpeftra ; and hence he infers, that all vifion is 

 owing to the aftivity of this organ. In the firft cafe, the 

 retina is not fo eafily excited into aftion by lefs irritation, 



S P E 



after having been lately fubjefted to greater. In the feconcf, 

 the retina is more eafily excited into aftion by greater irrita. 

 tion, after having been lately fubjefted to lefs. In the third, 

 a quantity of llimulus greater than natural excites the retina 

 into fpafmodic aftion, which ceafes in a few feconds : and a 

 quantity of ftimulus fomewhat greater than the laft excites 

 the retina into fpafmodic aftion, which ceafes and recurs 

 alternately. In the fourth cafe, the retina, after having been 

 excited into aftion by a ftimidus fomewhat greater than the 

 lall mentioned, falls into oppofite fpafmodic aftion. The 

 retina, after having been excited into aftion by a llimulus 

 greater than the laft mentioned, falls into various fucceffive 

 fpafmodic aftions. With a greater ftimulus, the retina falls 

 into a fixed fpafmodic aftion, which continues for fome 

 days ; and a quantity of ftimulus ftill greater, induces a tem- 

 porary paralyfis of the organ of vifion. For the illuftration 

 of the cafes, with mifcellaneous remarks, we muft refer to 

 the author's paper, ubifupra. 



SPECULA, among the Romans, were places whence a 

 good view might be had of what was doing at a diftance. 

 The word is particularly ufed to fignify watch-towers and 

 beacons. 



SPECULARES, in Natural Hiftory, the name of a 

 genus of fofGls of the clafs of the talcs. 



The word is derived from the \^zi\n fpeculum, a looiing- 

 glafs ; the bodies of this kind being naturally of bright, 

 gloffy, and poHlhed furfaces, and in the thicker maffes not 

 tranfparent, but reflefting the images of things. 



The fpeculares are talcs, compofed of vifibly feparate 

 plates of extreme thinnefs, and each fiflile again into a num- 

 ber of others yet finer. 



The bodies of this genus are the common Miifcovy talc ; 

 the fpccularis lucida fufca, or brown talc, a fpecies little in- 

 ferior to the former in beauty, and found in Germany and 

 England ; and the Jpecularit amethyjlina, called talc rouge, or 

 red talc, by the French. This is found in Mufcovy and Per- 

 fia, and no where elfe, as far as is yet known, and is often 

 imported into France in maffes, which are of a beautiful 

 purple : we have of it brought into England, but only in 

 thin flnkes, fit for the covering of miniature piftures. In 

 thefe flakes it has none of the rednefs natural to it in the 

 lump, but is the fineft and moft tranfparent of all the talcs. 



Pliny, and other of the ancient writers, as well as feveral 

 of the moderns, ufe the ieirm fpecularis lapis for that fpecies 

 of talc commonly known by the name of iftnglajs, or Muf- 

 covy glnfs. 



This has been a fubftance in great ufe among mechanics, 

 from the earlieft times of which we have any account. It is 

 found in broad flat mafles of ten or twelve inches in breadth, 

 and from half an inch to three inches in thicknefs ; and is 

 compofed in thefe of an almoft infinite number of broad and 

 beautifully even plates, or flakes, laid with a perfeft regu- 

 larity on one another, and feldom parting naturally from 

 each other, though by art they may be divided, almoft 

 without end, into broad and extremely thin laminz. Thefe 

 are very flexile and elallic, and make no effervefcence with 

 aqua fortis. By the laft of thefs properties they are diftin- 

 guifhed from the plated fpirs which fome have confounded 

 with them, and by their elalticity from all other foffile 

 bodies. 



It is found in many parts of the world. The ifland of 

 Cyprus abounds with it. It is very common alfo in Ruflia, 

 and has of late been difcovered to abound in the Alps, the 

 Apennines, and many of the mountains of Germary. It 

 IS imported in large quantities into England, and is ufed by 

 the lanthorn-makers inftead of horn, in their nicer works ; 

 by the painters to cover miniature piftures ; and by the mi- 

 crofcope 



i 



