CHAMBERS'S INFORMATION FOR THE PEOPLE. 



than it was supposed to be in his time, this esti- 

 mate is too large. 



As light advances from its source, it becomes 

 weaker, though not in the ratio of the distance. 

 If we take a board, a, a foot square, and place it 

 at any distance from a candle, it will be found to 



Fig. 2. 



hide the light from another board, b, two feet 

 square, at double the distance. Now, the latter 

 board having four times the surface of the first, 

 the light spread over it, when a is removed, must 

 be four times weaker. Thus we say, when the 

 distance is doubled, the intensity is one-fourth ; 

 similarly, when the distance is increased three 

 times, the intensity is diminished nine times ; and 

 generally, in whatever ratio the distance is in- 

 creased, the intensity is diminished in the square 

 of that ratio. 



This law is made use of in the construction of 

 photometers, or instruments to measure the in- 

 tensity of light. There are many varieties of the 

 instrument, but the most convenient and exten- 

 sively used is Bunsen's. A screen of white opaque 

 paper, which can be moved backwards and for- 

 wards on a divided scale, has a round grease- 

 spot in its centre. If a light is on the same side 

 of the screen as the eye, the central spot appears 

 dark on a white ground ; but if the light is on the 

 opposite side, then the spot is light on a dark 

 ground. If, now, we have a light on each side of 

 the screen, and if we shift the screen until the 

 whole of it appears of one uniform tint, it is clear 

 that the intensity of each light is as the square of 

 its distance from the screen. 



REFLECTION OF LIGHT. 



When a ray of light, CD, falls on a plane and 

 polished surface, as AB, it is bent back or reflected 



\ 



\, 



Fig- 3- 



in DE, making the angle EDA equal to the angle 

 CDB. If the surface is not plane, it is more con- 

 venient to draw the perpendicular DF, and then 

 the angle CDF is equal to EDF, or the angle 

 of incidence is equal to that of reflection. The 

 truth of this law may be experimentally shewn by 



242 



laying a plane looking-glass, AB, on a table, with 

 its front all covered except a small patch at D, 

 over which a plummet, FD, hangs. A candle- 

 flame, C, is now placed just as high above the 

 table as the eye of the observer is, and it will be 

 found on trial that the only point where the flame 

 is visible is a point, E, whose distance from the 

 plumb-line is the same as C's. 



If we draw the line ED backwards, and measure 

 off DG equal to DC, then an eye anywhere in the 

 line ED will see the light as if coming from G, 

 which is therefore called the image of C. If we 

 connect C and G by a straight line, meeting the 

 direction of the mirror in K, it will be found that 

 CG is perpendicular to AK, and that CK is equal 

 to KG. Hence the following rule to find the posi- 

 tion of the image of a luminous point formed by a 

 plane mirror. Draw a perpendicular from the 

 point to the mirror, and continue the line back- 

 wards its own length beyond the mirror ; the 

 extremity of the line is the image of the luminous 

 point. 



To trace the course of the pencil of rays by 

 which any point of an object is seen after reflec- 

 tion. Let A be 



any point of the 

 object ; B, the 

 image of A, de- 

 termined as be- 

 fore. From B 

 draw the diverg- 

 ing pencil of all 

 the rays which 

 can enter the eye 

 at E, and let 

 these rays meet 

 the mirror in 

 C. Join all the 

 points of inter- 



Fig. 4. 



section of the rays at C with A ; then the pencil 

 ACE is that by which the point A is seen. 



Since every point of an object has its own 

 image just as far behind the mirror as the object is 

 in front, the whole image must be of the same size 

 as the object, and its position independent of the 

 position of the eye. Suppose, again, AB to be a 

 mirror, with an object, CD, of twice the length, 



Fig- 5. 



placed in frdnt of it, and parallel to it, CA being 

 perpendicular to AB ; EF is the image; and being 

 twice the length of AB, the direction, FC, in which 

 an eye at C would see the point F, just passes 

 through B. The whole of the image would there- 

 fore be seen from C. And this explains the fact, 

 which puzzles many people, that a person can see 

 himself full length in a mirror of just half his 

 height. 



Reflection at a Concave Spherical Surface. The 

 shape of such a spherical mirror resembles that of 

 a watch-glass, silvered on the convex side, and 



