468 
MESSRS. ROSCOE AND THORPE ON THE MEASUREMENT OF THE 
termining this point, and accordingly we undertook a fresh series of observations at Catania 
in connexion with the Government Eclipse Expedition. The method of measurement 
employed in this, as in the above-mentioned series, was that already described by one 
of us *. It consists in exactly estimating the tint which uniformly sensitive paper coated 
with a thin film of silver chloride acquires on exposure to the action of daylight for 
a given time. The observations were made in the Garden of the Benedictine Monastery 
of San Nicola, at Catania. The position of the observatory was 
Eat. 37° 30' 12" N. 
Long. l h Q m 18 s E., 
as determined by Mr. Schott, of the United States’ Coast Survey. By barometric mea- 
surements it was found to be about 170 feet above the sea-level, a result confirmed by 
the independent observations of Mr. Schott. In order to secure as clear an horizon as 
possible, the insolating instruments were mounted on the roof of a portico in the garden 
overlooking the bay to the south and west. The place was well adapted to the work, 
the only intervening object of consideration being the Monastery itself, distant about 
100 paces to the east, the dome of which subtended an angle of about 10° with the plane 
of the paper. To the north lay Etna, about twenty miles distant, the summit of which 
subtended an angle of rather more than 4°. The influence exerted by these objects in 
cutting off the diffused solar radiation is far too inconsiderable to affect the results. The 
sensitive paper was exposed in the plane of the horizon, the insolating instruments being 
placed upon a tripod stand about 4 feet 6 inches in height. This stand was carefully 
levelled at the commencement of each series of observations, and the insolators were firmly 
clamped down after being adjusted due east and west, in which position they remained 
throughout the entire course of the observations. As we contemplated making conse- 
cutive exposures for direct and diffused chemical intensity every ten minutes during the 
progress of the eclipse from first to last contact, it was necessary to employ greater lengths 
of sensitive chloride-of-silver paper than we had hitherto used. The brass insolators 
were therefore proportionately increased in length, so as to allow of at least twenty-five 
observations to be made on a single strip. We assured ourselves, in the first place, that 
the paper could be kept for hours in the insolator exposed to the bright sunshine without 
darkening in the slightest degree, so long, of course, as the moveable brass slide covered 
the hole. This was doubtless in part due to the good radiating surface of the polished 
brass insolator, whereby the paper remained unheated even under the influence of direct 
sunshine ; for we have observed that the blackening which silver chloride paper contain- 
ing excess of silver nitrate suffers in time, however carefully it may be protected from 
the light, is promoted by increased temperature ; thus a strip of sensitive paper kept in 
the dark more rapidly undergoes alteration in the tropics than in England. 
Two insolating instruments were employed placed side by side ; by means of one the 
chemical action of total daylight was first observed in the ordinary manner, and imrne- 
* Roscoe, Bakerian Lecture, Philosophical Transactions, 1865, Part II. p. 605. 
