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 
efh 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 armg made of light wood, 
about four feet 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 k 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 I 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 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 71 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- 
