OPERATING-ROOM AND FIXTURES. 57 
window facing due south, and at such height 
that the eye of the operator is on a level with 
the eye-piece of the instrument when he stands 
erect. 
A shelf outside the window supports the helio- 
stat; and it is essential that this be accessible, for 
the purpose of adjusting the instrument, and re- 
moving it, for safety, when the operator has ceased 
working for the day. 
Some little difficulty arises in regard to the 
placing and adjusting of the heliostat when this 
has to be managed through the window, and it is 
desirable to have an outside balcony or platform 
by which the operator can conveniently get at it 
from without. This is not essential, however; and 
by a little ingenuity the heliostat-shelf and window- 
fixtures can be so arranged that the instrument 
can be passed through a shutter and adjusted in 
its position on the shelf outside. In this case, the 
microscope should not be placed near the centre of 
the window, but as far as possible to the left, so as 
to leave room on the right for a shutter through 
which the operator leans to manipulate the helio- 
stat. The upper sash of the window must be re- 
moved, and it is as well to remove both sashes and 
to close the window by a substantial and well-joined 
frame, which serves to exclude light and to sup- 
port the shelf inside for the microscope, and out- 
side for the heliostat. The right half of this frame 
carries the shutter which gives access to the helio- 
stat. The heliostat governor (Woodward’s) is at- 
tached to the frame in such a position that the axis 
