LIGHT. ABSORPTION OP. 



LIGHTS I X<1. 



at 



medium a* before the first incidence, we find the minimum deviation 

 when l - a' and a - i' ; the internal part of the ray is then equally 



1' - a 



inclined to both plane*. We have then * = -j , I = -j~, M = 

 ' "J" , which affords a simple method of determining the index 



of refraction of media capable of being formed into prisms. 



When light emanating from one point is refracted accurately to 

 another, if r, T represent the incident and refracted rays, and an are 

 of the curve by the revolution of which the refracting surface U 



generated, T, -r- are the sine* of the angles of incidence and re- 



<lr oV 



fraction (abstracting from their algebraic*! signs), therefore ^^,77 



-0 and r + r' = conit. : this equation belongs generally to a curve of 

 the fourth order, but if r be infinite, or the incident light parallel to 

 the axis, it gives a conic section, and if the arbitrary constant vanishes 

 the equation r = nr 7 represents a circle. If one surface be given, it is 

 easy to find a second oy which homogeneous light may be refracted 

 accurately to a given point. 



When light U incident on the generality of crystallised bodies, the 

 ray is refracted in two directions, one of which in uniaxal crystals obeys 

 the ordinary law of refraction, but neither in biaxal crystals. On the 

 theory of emission the forces cannot here be simply normal to the 

 bees of the crystals, but have a connection with the directions of the 

 axes of crystallisation, while on that of undulation the inertia of the 

 ether within such bodies, or else its elasticity, is different in different 

 directions, and the form of the wave-surface ceases to be spherical. 

 The further consideration of that subject will be resumed in the 

 article POLARISATION or LIGHT. 



The formation of foci and images by reflection and refraction follows 

 from the simple laws here discussed, for an account of which the 

 reader may refer to OPTICS ; the description of the instruments con- 

 structed to take advantage of the properties of light being given in 

 LEX* ; MICROSCOPE ; MIRROR ; OPTICS, PRACTICAL ; TELESCOPE. 



The laws of the reflection and refraction of light admit, as we have 

 seen, of being satisfactorily explained according to either of the two 

 theories respecting which the scientific world was once divided. But 

 while the undulatory theory explains the interference of light, and 

 traces into their most minute details the curious and complicated 

 phenomena of diffraction, no satisfactory account has ever been given 

 of these phenomena on the theory of emission. The remarkable 

 phenomena of polarisation again, so difficult to conceive on the theory 

 of emission, fall simply and naturally into their places on the theory of 

 undulations, when once the doctrine of transversal vibrations is ad- 

 mitted. And even to go no farther than the laws of reflection and re- 

 fraction, which entail the necessity of making opposite suppositions, on 

 the two theories, respecting the velocity of light within refracting 

 media, the crucial experiment of M. Foucaults already alluded to 

 (' Annales de Cbimie,' $ 3, torn. 41, p. 129) is decisively in favour of 

 the theory of undulations. 



It must be confessed that there is one phenomenon, perfectly xim|>ln 

 according to the theory of emission, which presents a serious diffi- 

 culty on the theory of undulations, namely, the astronomical phe- 

 nomenon of aberration, according to which a star appears displaced 

 from iu true position, in a direction towards the point towards which 

 the earth i* moving, by a small angle measured by the ratio of the 

 Telocity of the earth to the Telocity of light, multiplied by the 

 sine of the angle between two lines, drawn, one in the direction of 

 the earth's motion, the other from the earth to the star. The com- 

 mon explanation of this phenomenon involves the tacit assumption 

 that the rectilinear propagation of light is not affected by the motion 

 of the earth. We might have expected beforehand that, just as 

 a ship in moving through the water would dinturb in its neighbour- 

 hood the course of little ripples with which the surface might be 

 umered, so the earth in moving round the sun would push the ether 

 out of iu way, and thus disturb the course of progress of the little 

 tremor* of which light consists. To get jid of this difficulty, many 

 , Dr. Young's supposition, that the earth allows the ether to pass 

 r through pt as the wind does through a grove of trees. The 

 ilinear propagation of light, and consequently the phenomenon of 

 aberration, U however explicable without making this rather startling 

 mumitk^i, by supposing the motion of the ether consequent upnn 

 the motion of the earth to be of a particular kind. See several articles 

 in the ' Philosophical Magazine,' 3rd series, vols. 2" to 32. 



The production of colour* by ordinary refraction is considered in the 

 article* DurKMioN and RAINBOW ; for that produced by light passing 

 near the edges of bodies, and by interference, the reader may consult 

 IMrrRAcnoR and INTERFERENCE. For the colours of plates, see 

 UXDCLATORY THEORY. 



I.I' ,HT, ABSORPTION OF. [AMORrrioN or LIGHT.] 



LIGHT-BALLS, for military purposes, are hollow case*, either 

 spherical or in the form of cy Under*, terminated at each extremity by a 

 hniini'^*" : they are filled with a combustible compostion, and being 

 thrown, by night, in a burning state from mortan, or in some cases 

 from the hand, they serve to discover the working-parti or troop* of 

 I 



The spherical oases are made of canvas or cartridge-paper, rut into 

 eight equal gores of a proper form and the edges sewn together, a hole 

 being left for the introduction of the composition and the application 

 of a fuze. The oblong cases consist, frequently, of two hollow hemi- 

 spheres of iron, which are connected with each other by four slender 

 bars of iron attached to their bases, in positions parallel to the axis of 

 the case, and the whole U then covered with canvas : the entire length 

 is about 14 calibre of the piece of ordnance from which the ball is to 

 be projected, a mortar of one of the four different kinds, the calibre 

 varying from 4| inches to 10 inches. 



The composition consist* of pulverised saltpetre (6J Ibs.), pulverised 

 rosin (1J Ibs.), ground sulphur (24 IDS.), and linseed oil (4 lb.). The 

 dry materials, after having been passed through a sieve, are mixed with 

 the oil while the latter is in a boiling state ; and sometimes a small 

 quantity of mealed gunpowder is added. The oblong balls are filled by 

 passing the composition through a fuse-hole in one of the hemispheres 

 and are afterwards strengthened by cord wound about them. The tiro 

 U communicated to either kind of ball, at the time of being projected, 

 by means of a piece of quick-match in the fuse. 



Previously to besieging a fortified place, the works and the ground 

 about them are reconnoitred, usually by night ; the first trenches are 

 also traced and formed during the hours of darkness, and therefore the 

 defenders, at the commencement of the siege, prepare some mortars 

 charged with light-bolls : these balls being thrown beyond the glacis, 

 enable them to discover the operations of the enemy, and direct a fire of 

 shot against them. The assault of a breach is also frequently made by 

 night; and in this case light-balls are thrown by the defenders into the 

 ditches of the place for the same purpose. These may be made by 

 merely filling grenades with the composition above mentioned, and 

 they may be thrown by hand. 



Major-Qeneral Sir J. T. Jones states ('Journal 'of Sieges') that the 

 defenders of the towns which were besieged by the British army in 

 Spain threw light-balls in order to discover the operations of the 

 attack ; and that two or three men of the engineers' brigade were kept 

 in readiness to run up and extinguish them as they fell. That officer 

 adds that the men generally succeeded, in a few seconds, in smothering 

 them with filled sand-bags, or by shovelling earth over them. Some 

 casualties are stated to have occurred among the men so employed, 

 but the fire of the enemy being directed against them was thereby 

 diverted from the working-party often employed at only a few yards' 

 distance from the ball. On account of the great utility of such balls 

 for illuminating the ground occupied by the enemy, Sir J. T. Jones 

 recommended that grenades or other missiles should be connected 

 with them by pieces of chain, in order that, through the risk of 

 the explosion, men might be deterred from attempting to extinguish 

 the light. 



It may be added here that spherical cases of pasteboard or canvas 

 filled with a composition which while burning emits a great quantity 

 of smoke, are frequently discharged from mortars in order to conceal 

 a movement of troops from the view of the enemy : they are also 

 occasionally thrown from the hand either to suffocate the men 

 employed in the galleries of military mines or to compel them to quit 

 their work: these are called smoke-balls. The composition consists of 

 mealed powder (S lb.), pulverised saltpetre (1 lb.), pulverised seacoal 

 041b.), pitch (2 lb.), and tallow (4 lb.) : the pitch and tallow are 

 melted together, and the dry materials, after being sifted, are mixed 

 with the liquid. 



Ui.HT. liAKOMKTRIC. [BAROMETER.] 



l.ICHT, CIIKM1CAL ACTION OF. [CHEMICAL AFFINITY ; LUSIIT, 

 PHOTOGRAPHY.] 



LIGHT-EQUATION. In consequence of the time employed by 

 light to traverse the solar system, phenomena are not seen at the 

 exact moment of their happening. The first step in astronomical pre- 

 diction is the finding the absolute moment of time at which a pheno- 

 menon occurs ; the next is to apply a correction which gives the time 

 at wliich it is seen at the place for which the prediction is made. This 

 correction or equation is called the light-equation. This term is how- 

 ever principally applied to the correction which is necessary in the 

 case of eclipses of Jupiter's satellite*. 



l.lt.HTIlnUSKS and DUES. [TRIXITT HotJBB.] 



LIGHTNING. The general circumstances attendant on a thunder- 

 storm are familiar to most persons. It will however be useful to state 

 some of the most prominent, with a view to their explanation \\lirn 

 regarded as electrical phenomena. 



At first we nee light clouds forming with jagged edges, tho relative 

 motions of which are frequently opposite and variable. The atmos- 

 phere at the surface of the earth enjoys a stillness and < Mini, 

 accompanied with some elevation of temperature, as well as considerable 

 barometric and hygrometric changes, which produce on the animal 

 system the sensations of closeness, faintness, and oppression , and appear 

 even to the brute creation indicative of some awful and impending 

 changes. Some of these light clouds appear stationary, as if the forces 

 which produced contrary motions in the others made an equilibrium in 

 these. A low murmuring and continued sound of distant thunder 

 U soon heard, after which the lower region of the air is refreshed 

 with cooler but light breezes of uncertain direction. The calm is 

 resumed, but* the thunder-clouds are nearer, apparently larger, and 

 much b'lHtcr, .-ind thoir influence on the nervous system is felt by an 



