30 Heliostat for Fhotom-icrogra^phy . \^fonrnT^LuSyT^^^ 
to the extremity of the arm r, and is prevented from shpping from 
its position upon these rollers by being embraced by the loose 
collar/', also fixed upon the end of the arm f. This rod d must 
make an accurate right angle with the plate b. It terminates 
below in a crutch-shaped arm, e, seen in the drawing in profile, 
which supports the mirror g at the sides, allowing it thus perpen- 
dicular motion for adjustment for declination. The mirror may be 
clamped in any position by means of the screw g. The rotation 
of the rod e is effected by a band which connects a wheel on the 
arbor of the hour-hand of the clock with the fixed wheel e', made 
twice the diameter of the wheel of the arbor, so as to perform one 
revolution to the latter 's two, and therefore to turn itself once in 
twenty-four hours. 
The adjustment of the heliostat for time of day and declination 
of the sun is most easily efiected by the following means : — One 
arm of the crutch-shaped support e of the mirror is prolonged 
beyond the edge of the latter, and has attached to its extremity by 
a hinge a small cubic box, h, and which projects over the surface 
of the mirror, and is open only on the side represented in the draw- 
ing as facing the observer. Fig. 2 gives a perspective view of this 
box ; e shows the arm of the mirror-support to which it is hinged, 
furnished with a projecting knee, e', to prevent the box from turn- 
ing too far back upon the hinge. In the face of the box a, which 
fronts to the sun, is a fine slot, running exactly north and south, 
and upon the under-face 6 is a corresponding slot or line. Then 
to set the heliostat for time, it is only necessary to rotate the polar 
rod until the fine ray of sunlight passing through the slot in the face 
a falls exactly on the line on the under-face h. In similar manner 
the face c, which is turned toward the surface of the mirror, has a 
fine slot cut through it in a direction at right angles to that of the 
other slot, and upon the opposing surface d is a corresponding slot or 
line. Since these two slots lie in the direction of the polar axis, it 
is only necessary in order to set the mirror for declination, to turn 
it until the reflected beam from its surface which passes through 
the slot in the face c falls exactly upon the indicator slot in the 
face c?. After the adjustments are made, the little box may be 
turned up on its hinge so as to rest against the supporting arm e, 
and leave the reflected beam from the mirror unobstructed. Fig. 3 
shows in dotted lines the directions of the direct and reflected rays 
of the sun when the adjustments are corrected. 
If desired, the wheel e' may have a dial-plate on its upper 
surface, divided into twenty-four hours, and numbered from twelve 
above to twelve below, for the purpose of setting the instrument 
for time, d! in the drawing represents an indicator for reading the 
time on the plate attached to the arm d. A graduated arc attached 
to the edge of the mirror-frame may in like manner be used to 
