430 



NATURE 



\_March 28, 1878 



Fig. 4 represents a section of the [tube, and shows the 

 paths the different rays of light take, and shows how each 

 is reflected from side to side till they all meet in the eye. 

 The dotted lines and the rings projected beyond the tube 

 show how they appear to the eye. By studying this draw- 

 ing carefully, and trying cross cuts and slits in the card in 

 place of the single hole, you will get a very correct idea 

 of repeated reflection, and find the tube a source of con- 

 siderable amusement. 



Experiments in Dispersed Reflection 



Get a small piece of black velvet or cloth and take it to 

 a dark room where the heliostat will give us a slender 

 beam of sunlight. If this is not convenient use a common 

 beam of sunlight in a dark room, as in some of our former 

 experiments. Hold the velvet in the hand between the 

 fingers, and so as to leave the palm of the hand clear. 

 Turn back the coat- sleeve so as to expose part of the 

 white cuff, and then bring the velvet into the beam of 

 sunlight. You will observe nothing in particular, for the 

 black rough cloth does not reflect the light at all. Now 

 move the hand so that the spot of light will fall on the 

 palm. See what a pretty rosy glow of light falls on the 

 wall ! This is the reflected light from the hand. The 

 skin is rough, and the light is diffused and scattered 

 about, and instead of a bright spot of reflected light, as 

 with a mirror, we have this glow spread all about on the 

 wall and furniture. Now move your hand so that the 

 sunlight falls on your cuff. Immediately there is a bright 

 light shining on the wall and lighting the room with a 

 pale bluish- white glare. Move the hand quickly so that 

 the black cloth, the hand, and the white cuff will pass in 

 succession the beam of light. Observe how the different 

 things reflect the light in different degrees. The cuff is 

 the smoothest and whitest, and gives the brightest reflec- 

 tion ; the hand gives less light because it is less smooth ; 

 and the cloth, that has a very dark and rough surface, 

 gives no reflection at all, and the spot of sunlight, falling 

 upon it seems dull and faint. 



This experiment shows us something more in the 

 reflection of light. A piece of glass, the surface of water, 

 polished metals, ice, and all substances having very 

 smooth surfaces, reflect light in one direction. The linen 

 cuff also reflected light, but apparently in a very different 

 manner from the mirrors we have been using. 



Place a lighted lamp upon a table and lay a mirror 

 before it, and you can see a clear and distinct reflection 

 of the lamp and the flame pictured on the glass. Put a 

 sheet of white paper before the lamp, and you can see 

 only a confused spot of reflected light on the brightly- 

 lighted paper. Lay a freshly-ironed napkin or handker- 

 chief before the lamp, and even the indistinct spot of 

 light has disappeared, and the white cloth reflects light 

 equally from every part. 



These drawings are intended to show how light is 

 reflected from different surfaces. The first represents a 

 smooth surface, like glass, that sends all the beams in one 

 direction, because the points of reflection for the beam 

 are in the same plane. (See i, 2, 3, Fig. 5.) 



The second drawing represents a slightly-roughened 

 surface, like paper. Some of the points of reflection turn 

 the light one way, some another, and the beam of reflec- 

 tion is no longer formed of parallel rays. They are 

 scattered about, and the image they form is confused and 

 indistinct. In the third drawing we have a rough surface, 

 like cloth, and here the rays of the beam of reflection are 

 scattered in every direction, and we can see no image. 



It is in this manner that we are enabled to see the 

 people and things about us. The light of the sun or a 

 lamp falls upon them, and is reflected into our eyes, and 

 we say we see the objects. Very few things reflect light 

 so brightly that we obtain from them a reflected image of 

 the source of the light, and we generally see only dis- 

 persed and scattered light, that does not blind or dazzle 



the eye, and enables us to look upon these objects with ease> 

 and to readily see all their parts. 



The clouds, the water, the grass, rocks, the ground, 

 buildings, the walls inside, clothing and furniture, and 

 everything we can see, reflect light in every direction again 

 and again, and thus it is that all spaces, without and 

 within, are filled with light so long as the sun shines. At 

 night the sun sinks out of sight, and still it is light for 



Fig. 5. 



some time after, for the sunlight is reflected from the 

 sunset-clouds and the sky. 



Sometimes, upon a summer's day, when broken clouds 

 partly hide the sun, you will see long bars of dusky light 

 streaming from openings in the clouds. These long bars 

 are beams of sunlight shining upon dust and fine mist 

 floating in the air, and we see them because each speck 

 and particle reflects light in every direction. 



Experiment with Jar of Smoke 



Fig. 6 represents a large, clean glass jar, such as one 

 sees at the confectioner's. It is standing upon a black 

 cloth laid upon a table in a dark room, and on top of the 

 mouth is laid a postal-card, having a slit, one inch (25 

 millimetres) long, and t?^ inch (i millimetre) wide, cut in 

 it. Above the jar is a hand-mirror, so placed that the 

 beam of sunlight from the heliostat (or from a hole in the 

 curtain) will be reflected downward upon the postal-card 

 on top of the jar. 



This simple apparatus is designed to show how light is 

 reflected from small particles floating in the air. Set fire 

 to a small bit of paper and drop it into the jar. Place 

 your hand over the mouth of the jar, and in a moment it 

 will be filled with smoke. When the paper has burned 

 out, put the postal- card in place, so that the slit will be 

 in the centre of the mouth of the jar. Let the beam of 

 reflected light from the mirror fall on this slit. 



