653 
MR. H. E. ROSCOE ON A METHOD OF MEASURING THE 
close to the plate below the circular opening. In order to keep the paper and appa- 
ratus dry during rain, a glass shade is placed in wet weather over the insolation- 
apparatus, and the loss of light thus occasioned by reflection and absorption experi- 
mentally ascertained for each instrument. 
On unrolling, at the end of the day, the strip of insolated paper from the wheel (W) 
in a room illuminated by the monochromatic light of a soda-flame, the black disks of 
the hours are seen, and between each of these are found ten circles variously tinted, 
from that (probably scarcely visible) w r hich was exposed for 2 seconds, to that (perhaps 
too dark to read off) which was insolated for 30 seconds. Amongst these some one at 
least will be found to be of such a tint as to enable it to be read off on a graduated 
fixed strip, as described in my former communication*. 
In order to be able to read off each hourly observation quickly, the half of each of 
the tinted disks which is of about the right shade is punched out of the paper by a 
solid semicircular punch, which is worked by the foot, whilst the long strip of paper is 
held in both hands. One end of the long paper band is then placed in one of the 
spring clamps of the reading-drum f, and the band brought through the other clamp, 
so that the remaining halves of the tinted disks are held against the graduated fixed 
strip which is placed on the drum. By moving the drum on its horizontal axis, the 
various shades of the fixed strip are made to pass and repass each of the semicircular 
holes on the band ; and thus the point on the strip identical in tint with the remaining 
half of the disk can be easily ascertained, the reading being made in a darkened room 
by the light of a soda-flame. Each tint is thus read off ten times, and the mean taken as 
the result. 
II. On the Calibration of the fixed Strips and Standard Tints. 
The calibration of the fixed strips X can he advantageously effected, independently of 
the pendulum photometer, as follows : — The strip to be calibrated is gummed on to the 
reading-drum, and the points on this strip, of equal intensity to papers tinted by simulta- 
neous exposure to zenith-light under vertical cylinders closed at the top with differently 
sized diaphragms, are ascertained. For a calibration thus made six cylinders were em- 
ployed, each being 6 decims. long and 1 decim. in diameter, and blackened inside. On 
the top of each was fitted a metal plate perforated by a circular opening. These openings 
were of varying size, such that the relative intensities of the diffused zenith-light falling 
on the sensitive paper at the bottom of each cylinder were as follows : — 
Relative intensity. 
Cylinder 1 1*00 
„ 2 2-32 
„ 3 4-00 
„ 4 6-13 
„ 5 8-72 
„ 6 11-75 
* Phil. Trans. 186a, vol. elv. p. 610. f Ibid. p. 615, fig. 6. + Ibid. p. 610. 
