"^So^mTjanSr?^^^^^^ Seliostatfor Photomicrogm:phtj, 29 
passing through a circular hole in the centre of the square block n, 
that forms the top of the column or pedestal, it being connected to 
the foot, 0, by four uprights, p, p, jp, set a little apart, in which 
space hangs the plumb-line (Fig. 2) q, leveling being effected by three 
screws on the feet of the support. This pedestal, although 14 inches 
high, has to be placed on a stool set on the window-ledge and cut 
to its bevel ; when put in position, about a quarter of an inch of free 
space remains between the clock-case and the under-edge of the top 
window sash. If made of two parts only, it would have been less 
easy to get in position. 
The hand is passed through a sliding frame in the shutter to set 
the angle of the second mirror by the milled head and rod, which is 
less convenient than the method adopted by Dr. Curtis, of two rods 
working through apertures in the shutter and handled from within ; 
but in my case the shutter exposing a very large surface, it was 
found preferable to have the heliostat entirely disconnected. 
The loss of light occasioned by the employment of a second 
silvered surface, I find to be about in the proportion of 3 to 2, or to 
require 33 seconds' exposure, when 22 sufficed with a single mirror. 
These periods would doubtless be much reduced by using mirrors 
silvered on the upper surface. 
ly. — Heliostat for Photomicrography. By Lieut.-Col. J. J. Wood- 
ward, M.D., U.S., Army Medical Department, Washington. 
This heliostat is designed to throw a steady beam of sunlight in 
the direction of the south pole of the heavens. It consists essen- 
tially of a mirror, adjustable perpendicularly according to the 
declination of the sun, and attached to the south end of a rod set 
parallel to the axis of the earth. This rod is then rotated at the 
rate of one revolution in twenty-four hours, by having a wheel 
fixed upon it, which is connected by a band with the arbor of the 
hour-hand of an ordinary clock. 
Fig. 1 represents a side-view of the instrument. A is a triangular 
base-board mounted on three levelling screws, and furnished with a 
compass, a; b is a triangular plate of metal or wood, which supports 
the clock 0 and the right-angled arm d for carrying the polar 
rod E. This plate, b, is immovably fixed upon the base-board a, 
so as to make with it an angle which is the complement of the 
angle representing the latitude of the place where the heliostat is 
to be used. The polar rod e, which carries the mirror, is crank- 
shaped at the upper end, and rests by a pointed centre upon a 
depression in the bearing d, at the end of the fixed arm d. It is 
supported below by resting against two friction rollers, ff, attached 
