4o6 



NATURE 



\_March 21, 1878 



off when the instrument is not in use, and if the elastic 

 bands or rings are very strong^ they will answer perfectly. 

 The long mirror is to go on the movable piece at N, and 

 the small mirror stands on the shelf, facing the opening in 

 the board, at O. This mirror stands at the angle shown 

 in the next dfawing (Fig. 2), and the other mirror is ad- 

 justed to the sun at its various positions in the sky at 

 different seasons of the year. 



Here i^ a diagram showing the position of the handle 

 of the heliostat, and the mirror for different seasons and 

 in different parts of the country. The handle must be 

 placed on a line parallel with the axis of the earth, and 

 the four dotted lines give its position when the heliostat is 

 to be used in Boston, New York, Washington, and New 

 Orleans, This also causes the block of wood marked K 

 to have a slightly different shape, so that the hole through 

 it will be in the middle^ The dotted line marked "At 

 Equinox " shows the path of the light from the sun, and 



ShXe MomBLE Mirror. 



S'X5^ SnTioNARYMinnaH 



Fig. 2, 



the three dotted lines show the paths of the reflected light 

 as it passes from one mirror to the other. The position 

 of the movable mirror is also shown in the positions it 

 has at the summer and winter solstices. 



Fersi Experiment ivith the Heliostat^ 

 Choose a bright sunny day, and take the heliostat into 

 a room having a window facing the south. Raise the 

 sash and place the instrument in the window, and fasten 

 it there so that it will be firm and steady. Before closing 

 the window down upon it, move the larger mirror on its 

 axis till it reflects a beam of light into the small mirror. 

 Then turn the handle to the right or left, and a round, 

 horizontal beam of light will enter the room. When this 

 is done, close all the windows, so as to make the room as 

 dark as possible. To do this, shawls or blankets or 

 enamelled cloth will be found useful inside the curtains 

 and shutters. Then get a piece of cardboard, about 6 

 inches (15 "2 centimetres) square, and lay a five-cent piece 

 in the centre, and, with a knife, cut a hole in the card just 

 the size of the coin. Then fasten this, with pins or tacks, 

 over the opening in the heliostat. 



We have now a slender beam of light in a dark room. 

 Walk about and study it from different sides. See how 

 straight this slender bar of light is ; it bends to neither 

 the left nor the right, but extends across the room in an 

 absolutely straight line. As the suii moves, turn the 

 handle of the heliostat to keep the light in place. 

 iiere (Fig. 3) is a picture ©f adark room, in thewindow of 



which is the heliostat. In the centife of the piece of card- 

 board is the small hole where the light enters the room. 

 A boy is holding one end of a long piece of linen thread 

 just at the bottom of the hole in the card, and another 

 boy has drawn the thread out straight and tight, so that 

 it just touches the beam of light throughout its length. 



Were you to try this experiment, you would see that the 

 thread would suddenly be lighted up throughout its whole 

 length, and would shine in the dark room like silver. 

 Then if the boy allows the thread to become slack and 

 loose, or if he lowers it even a very little, it will disappear 

 in the darkness. If he raises and lowcs it quickly, it 

 will seem to appear and disappear as if by magic. 



This is a very pretty experiment ; but we must not stop 

 to look at its merely curious effects. Try it over several 

 times, and see if it does not show you something about 

 the beam of sunlight. Plainly, if the thread is lighted 

 up its whole length when it is straight, then the beam of 



FtG. 3. 



light rniust be straight also. Here we discover something 

 about light ; we learn that it has a certain property. 

 Our experiment shows that light moves in straight lines. 



Experiment with Cards ana a Lamp. 



Here (Fig. 4) is a picture representing three little wooden 

 blocks placed in a row upon a flat, smooth table, and 

 fastened to them are three postal-cards, so that they will 

 stand upright. At the end of the table is a small lamp. 

 This is all we need to perform another experiment, that 

 will show us the same thing we observed with the beam 

 of light from the heliostat. To make these things, get 

 a piece of wood 10 inches (25*4 centimetres) long, 3 

 inches (76 millimetres) wide, and \\ inch (37 millimetres) 

 thick, and saw it into 5 pieces, each i\ inches (64 milli- 

 metres) long. Next make three slips of pine, 4 inches 

 (10 centimetrcs) long, 3 inches (76 millimetres) wide, and 

 \ inch (4 millimetres) thick. Having made these, get 

 three postal-cards, and lay them flat on a board, one over 

 the other. Just here we need a tool for making small 

 holes and doing other work in these experiments ; and we 

 push, with a pair of pHers, a cambric needle into the end 

 of a wooden penholder or other slender stick, putting the 

 eye-end into the wood, and thus making a needle-pointed 

 awl. Measure off one-half inch from one end of the top 

 postal-card, and with the awl punch a hole through them 

 all, just half-way from each side. Lift the cards up, and 

 with a sharp penknife pare off the rough edges of the 



