92 CLIMATIC CYCLES AND TREE-GROWTH. 
THE AUTOMATIC OPTICAL PERIODOGRAPH. 
The present apparatus combines the two processes whose develop- 
ment has been described above. The second process developed is 
really the first one in the present instrument. 
The curve.—The curve is prepared by cutting it out in a thick 
coordinate paper. The space between the curved line and the base is 
entirely removed and the curve becomes represented by area. In 
order to make the density still greater, the paper is painted with an 
opaque paint so that the brilliant light passing through will come 
through only the curve itself and not the paper. A special window- 
shutter is made to occupy the lower 2 feet of the window, whose width 
is some 50 inches. The curtain can be drawn down to the top of this, 
excluding the light around the edges. This window-shutter has a door 
in the upper part to give access to the interior. Within this box is a 
sloping platform upon which a mirror 8 by 46 inches is placed. This 
mirror is about 35° from the horizontal position and when looked at 
from a horizontal direction it reflects the sky from near the zenith. 
On the side of this box toward the room is a slit 45 by 3 inches in size. 
This extends horizontally and is on a level with the mirror. Below this 
slit is a narrow groove for taking the lower edge of the curve paper and 
above this slit is a strip of wood on hinges, so that when the lower edge 
of the curve is placed in the narrow groove below, this hinged strip 
closes down on the top and holds the curve in place directly in front of 
the mirror. Looked at from a horizontal direction within the room, 
the curve is seen brightly illuminated by light from the sky not far 
from overhead. 
Track and moving mechanism.—About 7 feet from the curve the 
track begins and extends back 45 feet in a perpendicular direction. 
The track consists of 3 rails. The center rail is of uniform height and 
takes the single rear wheel, whose motion controls the movement of 
the film at the back of the camera. The right-hand rail is also uniform 
in height and supports one of the front wheels. The left rail is variable 
in height and supports the driving-cone, which serves as the other 
front wheel. The cone is 6 inches long and 3 inches in greatest diameter. 
It rests on a side rail whose elevation and distance from the center can 
be altered. The purpose of this particular mechanism is to vary the 
speed with which the camera travels along the track, for the time of 
exposure is approximately proportional to the square of the distance 
from the curve, and therefore when the camera travels from the near 
position to the far position it must slow down in rate as it goes along. 
The left rail, therefore, at the near position is close to the center and 
low down; in the middle and outer parts of the track it gets farther 
away and higher up, since the parts of the cone near the vertex travel 
on it. The axis of the cone carries a bevel gear meshing with another 
