MIRROR MISITRA. 



841 



his image ; so that it will cut across the cone which 

 comes from his image to his eye, half way between 

 its base and its apex ; the base of the cone is the 

 image seen, the apex is at the pupil of the eye, 

 where all the rays from the image are united in a 

 point. Concave mirrors are those whose polished 

 surfaces are spherically hollow. The properties of 

 these mirrors may be easily understood, when we 

 consider their surface as composed of an indefinite 

 number of small planes, all of which make a deter- 

 minate angle with each other, so as to throw all the 

 rays to a point. This point is called the focus of the 

 mirror, where an image of the object will be formed 

 in an inverted position. The distance of this focal 

 point from the surface of the mirror when the curva- 

 ture is moderate, will be equal to half its radius. Con- 

 cave mirrors are of great importance in the construction 

 of reflecting telescopes, in which they are commonly 

 called specula. (See Telescopes.) The employment 

 of concave mirrors in collecting the heat of the sun's 

 rays from the whole of its surface to a single point, 

 thus accumulating a very great degree of heat, for 

 the combustion and fusion of various natural sub- 

 stances that are infusible in the greatest heat capable 

 of being produced from ordinary fire, may be exem- 

 plified, among those of modern date, by the burning 

 mirror of M. de Villette. The diameter of this 

 metal speculum was three feet eleven inches, and the 

 distance of its focus from the surface was three feet 

 two inches. The composition of this metal was of 

 tin and copper, which reflects the light very power- 

 fully, and is capable of a high degree of polish. 

 When exposed to the rays of the sun, by doctors 

 Harris and Desaguliers, a silver sixpence was melted 

 in seven and a half seconds when placed in its focus. 

 A copper half-penny was melted in sixteen seconds, 

 and liquefied in thirty-four seconds; tin was melted 

 in three seconds, and a diamond, weighing four 

 grains, lost seven-eighths of its weight. The inten- 

 sity of heat obtained by burning mirrors or lenses, 

 will always be as the area of the reflecting surface 

 exposed to the sun is to the area of the small circle of 

 light collected in its focus ; thus the diameter of the 

 spot of light at the focus of Villette's mirror, was 

 0-358 of an inch, and the diameter of the mirror, 

 forty-seven inches : hence the area of these circles 

 was as 0-358 2 to 47 2 , that is, the intensity of the sun's 

 rays was increased 17,257 times at the focal point. 

 The loss of light occasioned in passing through the 

 medium of which the lens is composed, together 

 with that lost by reflection from the surface of 

 mirrors, must, however, be deducted from this 

 theoretical calculation. (For further information, see 

 Burning Mirrors.) Concave mirrors afford many 

 curious illustrations of -their peculiar properties ; for 

 example, when a person stands in front of a concave 

 mirror, a little further from its surface than its focus 

 lor half the radius of its concavity), he will observe 

 liis own image pendent in the air before him, and in 

 an inverted position. This image will advance and 

 rec ede with him ; and, if he stretch out his hand, the 

 image will do the like. Exhibitions have been 

 brought before the public, in which a singular decep- 

 tion was obtained by a large concave mirror. A man 

 being placed with his head downwards, an erect 

 image of him was exhibited in its focus, while his 

 real person was concealed, and the place of the 

 mirror darkened : the spectators were then directed 

 to take a plate of fruit from his hand, which, in an 

 instant, was dexterously changed for a dagger, or 

 some other dangerous weapon. Convex mirrors are 

 chiefly employed as ornaments in apartments. The 

 objects viewed in these are diminished, but seen in 

 an erect position. The images appear to emanate 

 from a point behind the mirror : this point, which is 



its focus, will be half the radius of convexity behind 

 their surface, and is called the negative or imaginary 

 focus, because the rays are not actually collected 

 as by a concave mirror, whose focus is called real* 



In the earlier periods, with which history makes us 

 acquainted, mirrors were made of metal : the Egyp- 

 tians, Greeks, and Romans made use of metallic 

 mirrors. Pliny, in his natural history, also mentions 

 the use of obsidian for this purpose. Gold and silver, 

 highly polished, were employed by the Romans for 

 mirrors, which were richly ornamented with precious 

 stones. The forms were various, but most commonly 

 oval or round. 



MISCHNA, or MISNA; the code or collection 

 of the civil law of the Jews. The Jews pretend that, 

 when God gave the written law to Moses, he gave 

 him also another, not written, which was preserved 

 by tradition among the doctors of the synagogue, till 

 rabbi Juda, surnamed the Holy, seeing the danger 

 they were in, through their dispersion, of departing 

 from the tradition of their fathers, reduced it to writ- 

 ing. The Misna is divided into six parts : the first 

 relates to the distinction of seeds in a field, to trees, 

 fruits, tithes, &c. ; the second regulates the manner 

 of observing festivals ; the third treats of women and 

 matrimonial cases; the fourth, of losses in trade, &c.; 

 the fifth is on oblations, sacrifices, &c. ; and the 

 sixth treats of the several sorts of purification. See 

 Talmud. 



MISDEMEANOUR, in law ; a crime of a lower 

 nature. Crimes and misdemeanours, properly speak- 

 ing, are mere synonymous terms, though, in common 

 usage, the word crime is made to denote such of- 

 fences as are of a deeper and more atrocious dye ; 

 while smaller faults and omissions of less consequence, 

 are comprised under the gentler name of misdemean- 

 ours only. 



MISERERE (Latin, have mercy) ; the name of a 

 celebrated church song, taken from the fifty-seventh 

 psalm, beginning, in the Vulgate, Miserere mei, Do. 

 "nine. The miserere forms part of certain liturgies, 

 and various great composers have taken it as a sub- 

 ject. The miserere of Allegri is particularly famous; 

 and this alone, sung by the papal choir, in the capella 

 Sistina, in the Passion week, would repay the trouble 

 of a visit to the " eternal city." 



Miserere is also the name given to pictures repre- 

 senting the dying Saviour. 



A terrible disease, produced by an obstruction of 

 the bowels, is also called by this name. 



MISERICORDIA (mercy; in Greek, tins) was 

 personified as a deity. She had a celebrated altar in 

 the market-place of Athens, constituting an asylum. 



Misericordias Domini is the name given to the 

 second Sunday after Easter, because the mass for this 

 day begins with Misericordias Domini cantabo in 

 (eternum. 



Misericorde (French) was also the name of the dag- 

 ger of the knights in the middle ages. Fauchet de- 

 rives its names from its putting men out of pain when 

 irrecoverably wounded, or from the sight of it causing 

 the vanquished to cry out for mercy. 



MISHNA. See Mischna. 



MISITRA, or MISTRA; a city of Greece, in the 

 Morea, capital of the department of Laconia. It IKS 

 nearly a league from the ruins of Sparta, which have 

 supplied materials for its construction. Before the 

 Egyptian expedition to the Morea, it contained GOOO 

 inhabitants and several churches, literary institutions 

 and manufactories ; it is now a heap of ruins, in- 

 habited by about 150 families. 



* The reflecting surface of a cylinder has been occasionally 

 used in optical ainu^-mcnls for giving to anamorphoses (dis- 

 torted or deformed pictures) regular shnpes, when reflected from 

 such surface 



