CONCAVE AND CONVEX MIRRORS.] UNDULATORY FORCES. LI G HT. 



surface is that which is to be coated with mercury. Re- 

 flectors of this kind must form a portion of a sphere ; and 

 a watch-glass, of the ordinary form, illustrates the shape 

 of such a reflector. The "silvering" process is easily 

 performed ; and for this purpose the glass must be care- 

 fully cleaned, so as to be quite free from dust, spot, or 

 dirt : a piece of clean tinfoil is to be pressed on one of 

 its surfaces, so as to evenly cover every part; a little 

 mercury is then poured on to the tinfoil, and evenly 

 spread by means of a hare's foot, the hi'.irs of which an- 

 swer the purpose extremely well : a piece of thin tissue- 

 paper may then be laid on the foil and mercury ; and 

 gentle pressure is to be applied, so as to force off the 

 superfluous metaL The amalgam will adhere to the 

 glass surface, and thus form a good reflecting coat. 

 Either concave or convex reflectors are thus made ; and 

 although inferior to metals in their reflecting power, they 

 will illustrate many interesting laws and applications 

 of light. The most convenient way of arranging two 

 reflectors is by placing them on stands, allowing them to 

 slide on a rod, to which they can be fixed in any desired 

 Fig. 62. position by means of a binding-screw. 



Two should be obtained ; and their ap- 

 pearance is illustrated in the annexed 

 engraving. Referring to our remarks 

 on curved reflection,* it will be found 

 that we there enter fully into the ex- 

 planation of such matters as focal dis- 

 tance, (fee. ; aud these data, in reference 

 to any mirror which the student may 

 employ, must be ascertained by him 

 experimentally, according to the rules 

 we have laid down, and which we shall 

 not repeat here. Such having been 

 found, the following uses of curved re- 

 flectors may easily be examined, and 

 several interesting experiments may be 

 tried. 



If a concave mirror be so placed as to receive the rays 

 of light and heat from the sun, such will be condeu.sed 

 at the focal point, and powerful heat and light will bo 

 there produced. It does not matter whether the re- 

 flector is entirely spherical ; for if made-up of a number 

 of reflecting planes, the same effect is produced. Very 

 powerful " burning-glasses" have been thus constructed, 

 by building up a number of small plane reflectors in a 

 concave curved form. By means of these the most re- 

 fractory inutal may bo fused ; aud other substances which 

 resist the heat of the most powerful furnaces are con- 

 verted into the liquid state. Such reflectors have been 

 in constant use in lighthouses, for the purpose of casting 

 the rays of a lamp to a considerable distance. Perhaps 

 the most remarkable instance of this sort was that of the 

 reflection of the lime light in Scotland, during the survey 

 made. On one occasion, the rays reflected by a concave 

 mirror were distinctly perceived at a distance of ninety 

 miles from their source. By similar means, the light 

 produced by Holmes' magnetic arrangement was cast 

 across the English Channel from the South Foreland 

 lighthouse, and seen on the French coast, shining witli 

 great brilliancy. Concave reflectors, combined with 

 IIMISOS, are similarly employed ou railways, as "signal 

 lights ;" and in any case, where a powerful aud concen- 

 trated light is required, their use is of great value. 



If an object be placed somewhat beyond the centre of 

 curvature of a concave mirror, an inverted imago of it 

 may bo seen in the air. This effect can bo applied for a 

 variety of amusing pur[M>su.. Thus, a jug of water, if its 

 contents bo poured out, will seem to be filling. An 

 excellent plan of illustrating this amusing fact, is that 

 of allowing thin smoke, as from smouldering brown 

 paper, to pass between the observer aud the mirror, when 

 the inverted imago will bo distinctly porcoived. The 

 objoct is to be placed a little beyond the focus of the 

 mirror in such experiments : the proper position is easily 

 :aim:<l, however, by a trial or two. A person 

 standing beforo a concave mirror, beyond the centre of 

 curvature, will see an inverted and smaller image of 

 See ante, p. 44, tt *tq. 



himself ; and by moving to and fro, the image will appear 

 to advance till the centre of curvature is reached : 

 passing that, the image will seem to recede from tha 

 observer. 



Some amusing optical illusions may be effected by 

 means of concave mirrors ; one of which we shall here 

 illustrate, and leave to the ingenuity of the reader to 

 extend them in any other form he pleases. 



If an object be placed, as we have just described, before 

 a concave mirror, the spectator will see an inverted image, 

 when placed in certain positions. If, however, the object 

 be inverted, then an upright or vertical image will be pro- 

 duced. Availing ourselves of tliis fact, and placing before 

 the mirror a screen having an opening of less diameter 

 than the mirror, a spectator will see an image of any object 

 placed between the screen and the reflector, apparently 

 suspended in the air. Cameos, statuary, aud other 

 matters may be employed for the purpose ; and for the 

 amusement of young people, objects of a comic character 

 may be employed. The arrangement will be fully under- 

 stood by inspecting the annexed engraving ; in which a 

 Fig. 63. 



represents the concave reflector; 6, the object which 

 must be inverted ; c, the screen ; d, the aperture therein ; 

 e, the image formed in the air ; and /, the eye of the 

 spectator. (See Fig. 63). The use of smoke, as before 

 recommended, improves the effect ; and if the desire be 

 to startle or surprise, the mirror and object should be 

 placed in one room, and the observer should look through 

 an opening in a doorway, from the apartment in which 

 ho is placed, adjacent to tnat containing the mirror. 

 The cause of the imago will thus be entirely concealud 

 from him ; and tlio effect on his imagination will bo pro- 

 portionably increased. 



From our observations on convex mirrors, t the reader 

 will be prepared to find, that an image cast thereon will 

 bo magnified, at certain positions, instead of being dimin- 

 ished, as is the case when concave reflectors are similarly 

 employed. On bringing an object near to a convex re- 

 flector, its imago increases in size until the object aud 

 the reflector come in contact. This may be observed by 

 watching the effect on the image of the face, whilst 

 approaching or receding from a polished teapot, whoso 

 body, generally speaking, is of a convex form. Some 

 concave and convex mirrors, which are sold by tho opti- 

 cians, have an amusing effect on the minds of persons 

 looking thereon, especially if they are ignorant of tho 

 laws of optics. These mirrors are constructed so as to be 

 viewed both widthways and lengthwise ; and they respec- 

 tively produce a ludicrously enlarged image of the faco 

 and features generally. These were often used as mirrors 

 for the drawing-room, some years ago ; and, perhaps, 

 mauy of our readers may liave seen them ill houses 

 furnished after the old fashion. 



By means of concave mirrors of a cylindrical form, 

 some very curious optical effects are produced, similar to 

 those we have just named. Objects are painted, say of 

 a landscape, &c. , presenting an irregular or distorted 

 form when seen by the eye. If, however, such are 

 viewed by means of the mirrors we have been describing, 

 they at once present a definite form. Such are called 

 anamorphoses, on account of the peculiar change of oilcct 

 which is produced by the arrangement. 



SPECTACLES. 



WB have already mentioned, incidentally, instruments of 

 tliis kind :J we shall, however, enlarge on the subject, 



t Sec unte, p. 45. 



4 Ante, p. 49. 



