RECREATIVE SCIENCE. 



217 



It in i-inuriu. tit to regard a sunbeam as an assemblage of 

 waves, one only of which may bo called a ray. Light distri- 

 butes itself on all sides from a self-luminoua body, like radii 

 drawn from the centre of a circle; it must therefore follow that 

 each ray purnuos a straight lino, although diverging from the 

 one by the side of it. As all the single rays forming a beam 

 of li^'ht diverge from each other, unless altered by special 

 optical contrivances, they may be represented by a fountain jet 

 in which the streams of water are forcod out in straight lines, 

 not parallel but diverging from each other. (Fig. 3.) 



If the hand bo hold 

 across the fountain 

 at A, all the water 

 issuing from the jet 

 may be directed or 

 thrown off to the 

 side ; at B only a 

 certain number of 

 the streams of water 

 impinge upon the 

 hand ; and at c the 

 number would bo 

 still less. By ana- 

 logy, the metal jet 

 may represent a lu- 

 minous point; the 

 streams of water the 

 rays of light issuing 

 from it. At A all 

 the rays strike upon 

 the object, which 

 would be highly 



illuminated ; at B the object could not look so bright, because a 

 smaller number of rays illumine it ; and at c the light would 

 be still more feeble. In consequence of this divergence of the 

 rays from each other, light decreases in intensity ac the distance 

 from the luminous point is increased ; and this fact is called the 

 law of inverse squares the greater the distance, the less light, 

 and vice versA. The law is proved by observing the shadow 

 cast by an opaque body held in the beam of light ; the more 

 intense the light, the darker will be the shadow. A board, one 

 foot square, held at a certain distance from a bright light, 

 will completely 

 throw into shadow 

 another board, 4 

 feet square, held at 

 twice the distance 

 from the luminous 

 source ; and this, 

 again, a third board, 

 9 feet square, placed 

 at three times the 

 distance from the 

 source of light. 

 The law is easily 

 reduced to figures : 

 the first distance ia 

 taken as unity, or 

 1 ; at the second dis- 

 tance, the square of 

 2 being 4, the shadow 

 is spread over four 

 times the area, and 

 is four times less 



intense on each square foot ; at the third distance, the square 

 of 3 being 9, the shadow is spread over 9 square feet, and is 

 nine times less intense on each square foot, and so on to any 

 required distance. The law is demonstrated in a most amusing 

 manner by placing a very bright point of light, such as that 

 obtainable from the lime-light, on the floor, and at a certain 

 distance, say 10 feet, from a transparent screen. Place a boy 

 at 2 feet distance from the lime-lignt, when his body, intercept- 

 ing all the rays, will cast a shadow as high as the screen. 

 Now request him to walk double the distance, or 4 feet from 

 the light : and the shadow will be reduced considerably in 

 height, and, if measured, would bo found in accordance with 

 the law of inverse squares ; and so on, as the boy walks away 

 to a distance of 6, 8, or 10 feet, until ho touches the screen. 



Fig. 5. A. SPHISOIKO UP TO THE CEIUWO. 



At thin position the shadow of his body dwindles down to a 

 very small one, again increasing; in height as he returns to bis 

 original station, close to and in front of the light If three 

 persons are stationed at unequal dUtanoes from the light, three 

 shadows are oast upon the screen, c, and the shadow of the on* 

 nearest the sheet may be so small that it is easily seen below 

 the Brobdingnagian stride of the shadow of the figure nssjeet 

 the light, who may place the shadow of his arm apparently 

 on that of the intermediate figure. (Fig. 4.) 

 The shadow effect is still more ludicrous when the figures jump 



over the light; and 

 they appear to spring 

 up to the ceiling if 

 they jump away from 

 the screen and over 

 the light, or to come 

 down from the ceil- 

 ing to the floor when 

 they spring from 

 the back of the light 

 towards the snram 

 (Fig. 5.) Wherever 

 these opaque bodies 

 out the rays of light, 

 an equivalent to a 

 solar eclipse is the 

 result; the dark sha- 

 dow moves across 

 the screen as that of 

 the moon does upon 

 the earth ; and thin 

 is called the path of 



the shadow a direction always known, and to which astrono- 

 mers go when they wish to observe a total eclipse oi the sun. 

 It is on this principle that photometers, or light-measurers, are 

 used. 



Two unequal lights will have the effect of casting two dif. 

 ferent shadows, one of which will be stronger than the other, 

 if the lights are placed at the same distance from the wall. 

 A lighted candle and the flame of a moderator argand 

 lamp will cast two shadows of a stick held in front of 

 them. One will be darker t-h" the other, but if the candle is 



gradually brought 

 nearer to the 

 wall or sheet of 

 paper on which the 

 two shadows are 

 thrown, the dis- 

 tances of the lights 

 may be so adapted 

 that the two sha- 

 dows of the stick 

 shall be equally 

 dark. It is now 

 only necessary to 

 measure the dis- 

 tances oi the two 

 lights from the 

 wall, and after 

 squaring the two 

 measurements, 

 the greater may 

 be divided by the 

 lesser one, when 



the quotient will give approximately the illuminating powei 

 of the lamp as compared with that of the candle. Thus, 

 suppose the lamp to be twelve feet away from the screen 

 and the candle four, then 12 X 12 = 144, 4X4 = 16, and 

 144 -T- 16 = 9. The lamp gives nine times more light than the 

 candle. 



When bodies are illuminated they scatter or reflect the rays 

 of light from every part of their surface. If we draw straight 

 lines from each part of an arrow, and imagine these straight 

 lines to be reflected rays of light entering the pupil and lenses 

 of the eye, they necessarily form a cone, the point of which will 

 impinge upon the retina. This fact will be considered more 

 fully in the next paper, and especially in connection with the 

 laws of reflection. 



