386 Mr. Potter on FrcsneVs Experiment of Interferences 



its place by steps screwed over the pivots. An aperture is cut 

 straight through the centre of the cylinder, which in the axis at 

 ef'is circular, of a little more than half-inch diameter, but to- 

 wards the surface it is extended longitudinally : through it the 

 sun's light falls on the small convex mirror k. This mirror is 

 attached, as in the figure, to the arm^ h, made of light wood, 

 about fourfeet long, which fits tightly in a lateral direction in 

 a groove g, fig. 2, but can be moved in a plane passing through 

 it and the axis of the cylinder, so as to place the mirror 

 in the sun's light shining through the hole. The axis of the 

 cylinder is fixed parallel to the earth's axis, when the appa- 

 ratus is finally secured in the window-shutter of a room with 

 a south aspect. The form of the aperture allows for the sun's 

 declination at different times of the year, by which means, 

 and the motion of the arm, the sun's light can be made to 

 fall on the mirror ^^ at all seasons; and to follow the sun's 

 daily motion we have only to push the arm so as to turn the 

 cylinder about its pivots. Every part is made to fit just so 

 tight that the apparatus remains in any position in which it 

 is placed. The two mirrors are placed as at / ??2, so that the 

 sun's light just passes their edges when it falls on the mirror 

 k; by this arrangement we make it certain that the light 

 received by the two mirrors is that which is reflected nearly 

 directly by the mirror k, and therefore with very little aber- 

 ration. The two mirrors are attached to a piece of wood, 

 which, having a hole in the line of contact of the mirrors, is 

 moveable about a thick wire which fits tightly into this hole. 

 This piece of thick wire is bent at right angles in three places, 

 as seen in fig. 1, and forms a very convenient universal joint, 

 which allows the mirrors to be turned about in any direction 

 required : at its lower part it is pushed through a hole in a 

 piece of wood fixed to the side of the arm g h. The dotted 

 lines n I m k, are intended to show the course of two rays 

 from the mirror to their interference at n in the focus of the 

 eye-lens. 



All the parts of the apparatus should be well blackened, 

 and a piece of black velvet (as the most perfect black) placed 

 behind and about the mirror. It is also desirable to have a 

 tube of blackened paper placed on the arm near k, through 

 which the sun's light may shine, but which will prevent the 

 stray light from the mirror k injuring the darkness of the 

 room. 



Those who understand analytical geometry will find the 

 readiest way of fixing the cylinder parallel to the earth's axis, 

 to be that of calculating the lines of intersection of the board 

 in fig. 2 with the plane of their window-shutter: great accu- 



