March 28. 1878] 



NATURE 



429 



of any piece of smooth glass will reflect light, and our 

 experiment is designed to show a still more curious 

 matter. 



b* Fig. 3 represents the single beam reaching the point 

 of incidence on the outside of the mirror at O, and 

 reflected to the wall at i. Part of the light goes through 

 the glass to b, and here is another point of incidence, 

 and a new beam of reflection is thrown through the glass 

 to the wall at 2. If you look at the reflections on the 

 wall, you will see that the second spot of light is the 

 brightest. This comes from the quicksilver, for, as this 

 is a better reflector than the glass, it sends out a brighter 

 beam of reflection. When this second beam of reflection 

 passes through the glass, a part of its light is reflected 

 from the under side of the surface, and is turned back 



Fig. 3. 



against the quicksilver again. Once more it is reflected, 

 and a new beam of reflection makes number 3. The 

 drawing shows the path these beams of light take in 

 the glass, and the quivering spots of light on the wall 

 show how one beam of light may be reflected again and 

 again in different directions. If the reflector was perfect 

 and returned all the light, these multiple reflections might 

 be repeated many times over ; but every time light is 

 reflected from any bright surface a part of the light is 

 lost, and thus each reflection grows fainter and fainter 

 till the light is spent. Look at the multiplied reflections 

 on the wall, and you will see that the first reflection from 

 the glass is bright, and that the second, from the quick- 

 silver at the back of the glass, is brighter still ; and that 

 the others grow fainter and fainter till all the light is 

 spent, and the reflections disappear. 



Second Experiment in Multiple Reflection 



Light a lamp and place it on a table, and get the two 

 postal-cards and the blocks that we used in the experi- 

 ment in reflection. With a sharp knife cut a slit in one 

 card, just at the pin-hole, about \ inch (19 millimetres) 

 long and ^ inch (i millimetre) wide. Then place this 

 card close to the lamp, as in the other experiment, and 

 set up the other card about fifteen inches away from it. 

 Then lay a looking-glass on the table between the two. 

 Look at Fig. 2, and arrange the cards as there repre- 

 sented, and put the mirror in place of the blackened 

 glass on the blocks. On looking through the small 

 hole in the postal card (marked B in the drawing), 

 you will see in the mirror several bars of yellow light, 

 placed one over the other. Again we have an instance 

 of multiplied reflection. Instead of seeing the reflections 

 thrown upon the wall, we can look down upon them and 

 see them, just us they stand, each at its point of incidence 

 on the glass and the quicksilver. Study these brilliant 

 bars of light, examine the diagram carefully, and you will 

 readily see. that this experiment simply exhibits in a 

 different manner the same thing that we saw in the last 

 experiment. 



Experiment with Mirror on Pulse 



Get a small bit of looking-glass, about an inch (25 

 millimetres) square, and some wax. Warm the wax in 

 the hand till it is soft, and then make three small pellets 

 about the size of a pea. Put one of these on the back of 

 the little mirror, near the edge and half-way between two 

 comers. Place one at each of the opposite corners, so 

 that the mirror will have three legs or supports placed in 

 a triangle. Put the heliostat in place, and bring a small 

 beam of sunlight into the dark room. If this is not con- 

 venient any beam of sunlight in a dark room (as in former 

 experiments) will answer. 



Turn back your coat- sleeve, and, while standing near 

 the beam of light, place the little mirror on the wrist, with 

 one of the wax legs resting on the pulse. Then bring the 

 arm into the beam, so that the light will fall on the 

 mirror. Hold the arm steady, and watch the spot of 

 reflected light thrown upon the wall. See ! It moves 

 backward and forward with a curious, jerking motion. It 

 is like the ticking of a clock, or like the bending of one's 

 pulse. It is the motion of your pulse. The mirror moves 

 with the pulse, and the beam of reflection thrown on the 

 wall moves with it, and, though this movement is very 

 slight, the reflection on the wall moves over a space of 

 several inches, and we can see it plainly. In our first 

 experiment in reflection we learned that when a mirror 

 was moved to the right or left, the beam of light reflected 

 from it moved also to the right or left, and each time through 

 twice as great an angle as the mirror. 



This experiment is a wonderfully interesting one, and 

 may be tried with a number of boys or girls, and each 

 may see the peculiar beating of his or her pulse pictured 

 on the wall in the most singular and startling manner. 

 If any of the persons whose pulse-beats are thus exhibited 

 get excited, laugh at the exhibition, or are in any way 

 disturbed, the change in the movement of their pulse will 

 be quickly repeated on the wall, where a hundred people 

 can see it. 



Experiment with Glass Tube 



Procure a glass tube, about | inch (19 millimetres) in 

 diameter and 12 inches (30*5 centimetres) long, and paint 

 the outside with black varnish. If this is not convenient, 

 cover the tube with thick black cloth, and fasten it down 

 with mucilage, taking care to have the cloth square at 

 the ends. Punch a hole in a postal-card with the sharp 

 point of a pair of scissors, and with a knife make the 



Fig 4. 



ragged edges of the hole smooth. Hold the card at one 

 end_,of the tube." so that the hole will come just at the 

 centre of the opening, and then, while facing a window or 

 a^bright lamp, look through; the tube with one eye, and 

 you will see a spot of light surrounded by a number of 

 beautiful rings. 



Here we have another example of multiplied reflection. 

 The light entering the tube through the hole in the card 

 falls on the smooth surface of the interior of the tube, 

 and appears to the eye in the form of rings. 



