RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 47 
and interruption of actual sunshine, or, when the sun is not visible, of the illumina- 
tion of that point in the clouded sky behind which the sun is situated. 
119. To accomplish the first of these objects, it would be necessary, in strictness, 
to expose the photographic paper destined to receive the register, to a full uninter- 
rupted view of every part of the sky, that occupied by the sun’s disc alone excepted, 
which would not be very easy to accomplish, without a somewhat complicated appa- 
ratus, to cut off the direct solar rays, by a moving screen of the requisite diameter. 
But if we consent to sacrifice the light of the zone of the heavens traversed by the 
sun in its diurnal motion, and such other small portions as may be intercepted by a 
slight frame-work, nothing is more easy. Let a weather-proof box or case be con- 
structed, within which a clock-motion shall carry round, once in twenty-four hours, 
a cylindrical barrel having its axis horizontal, and its upper surface close to the 
under surface of the top of the box, which at this point must be thin, and pierced 
with a narrow oblong window bevilled off at the edges so as to admit light from 
every part of the sky (but what is intended to be screened) to fall on that part of the 
circumference of the cylinder which travels close beneath it. The cylinder then 
being daily covered with a strip of sensitive paper, will of course register the total 
illumination which the slit admits to fall upon the paper, that is to say, the light ot 
the hemisphere plus the direct sunshine if the latter be not intercepted. To accom- 
plish this nothing further is needed than to adjust a light cylindrical hoop of brass to 
slide smoothly but stiffly up and down along three straight, parallel steel or brass rods, 
of no greater diameter than is necessary to give the requisite strength ; the direction 
of which shall be towards the pole of the heavens, and whose places shall be such 
that the axis of the hoop shall pass through the middle of the exposed area of paper. 
The upper ends of these rods should terminate in and be connected by a strong brass 
ring sustained by a vertical pillar ; the lower must be fixed to the pedestal or other 
support of the clock and its case, on which also the pillar above mentioned must be 
fixed. It is evident then that at any time of the year, or whatever be the sun’s de- 
clination, if the hoop be adjusted and clamped so as to throw its shadow across the 
window at one moment of the day, it will do so throughout the whole day, and the 
proper place for each day in the year may be marked on the rods. 
120. This contrivance, though convenient, and necessarily effectual, is not that 
which I ultimately adopted, which is as follows. A spring watch belonging to a 
’Sgravesande’s heliostat (as constructed by Robinson) has a hollow axis for the hour- 
index passing completely through it, from face to back, so as to permit a steel axis, 
removable at pleasure, to traverse it, and issue below. It is mounted on a stand so 
as to admit of adjustment both in altitude and azimuth, the former by a small di- 
vided quadrant and vernier, and so placed (according to the intention of its original 
use) as to have the upper surface or dial-plate of the watch parallel to the equator, 
and of course the axis of the hour-index directed to the pole. On this axis, and at 
the upper surface of the watch, was fastened a disc of sensitive paper, attached to a 
