[PLASKETT-DELURY] DETERMINATION OF THE SOLAR ROTATION 109 
with its axis pointing to the north pole and driven by clockwork, so 
that it directs the sun’s beams southward to another twenty-inch plane 
mirror, which in turn reflects the sunlight northward to an eighteen- 
inch concave mirror. This mirror reflects the light southward into the 
solar laboratory, focussing the image at about 80 feet. The primary 
mirror with its clockwork is mounted on an iron truck which may be 
rolled on iron rails, supported on a cement pier, eastward or westward, 
so that in the morning it may be moved to one side and in the afternoon 
to the other side to get more nearly normal reflection. The secondary 
mirror is mounted so that it may be rolled in a northerly-southerly 
direction to allow for the seasonal changes in declination of the sun. 
By rotating and tilting this mirror the sunbeam may be thrown fairly on 
the concave mirror. The concave mirror may be rolled back and 
forth to focus the image at any desired place in the laboratory. To 
this mirror are clamped two arms operated by screws, by turning which 
the mirror may be tilted in two directions giving motions to the image 
in vertical and horizontal directions. By means of cables running 
through pulleys to turning drums! in the laboratory, these motions 
may be given from beside the image so that it may be placed in any 
desired position, and any “drifting” of the image due to irregularities 
in the driving of the clock or to atmospheric disturbances may be 
corrected. A similar device is being installed so that the focus may 
be altered from a point in the laboratory. The primary mirror is 
covered by a white window blind, operated by means of a string leading 
into the laboratory, and used to protect the mirror, when not in use, 
from the sun’s heat which distorts the surfaces of the mirrors, thus 
spoiling the definition of the image. The latter is seldom perfect on 
account of the unsteadiness of the atmosphere, but it is not greatly 
disturbed in the coelostat house on account of the louvres (covered 
with canvas in the winter) which induce circulation thus preventing 
stratification in the passage. 
By means of this telescope the sun’s image is thrown on the face 
of the 23-foot spectrograph, a detailed description of which has already 
been given along with some results obtained with the first grating 
tested,” and the measurement of 80 lines on one of the rotation plates 
made by it. The optical parts of the spectrograph are:—A slit with 
metal jaws provided with a micrometer, for adjusting its width, reading 
to thousandths of an inch; a six-inch collimating lens of 22 ft. 10 in.- 
focal length for yellow light; and a plane reflecting grating ruled 
on speculum metal. The parts are arranged on the “ auto- 
! De Lury, Jour.-Roy.-Astron. Soc., Can. V, 33-35. 
? De Lury, Report of the Chief Astronomer for the year ending Mar. 31, 1909, 
pp. 251-256. 
