1453 
inquiry, that for temperature differences between the solderings not 
greater than those produced by full sunshine, the electromotive force 
of the thermopile could be considered strictly proportional to the 
intensity of irradiation. The deflections of the Sirmens and Harske 
galvanometer were observed visually, by examining the positions of 
a bright index on a transparent scale. With a permanent shunt of 
16 Ohms the instrument was just dead-beat; one millimeter deflection 
then corresponded to 10-* Amp. The deflections were proportional 
to the current. The observer had the resistance box close at hand, 
in order to keep the image on the scale, and marked the epoch of 
each reading by means of a doublehanded chronometer, one hand 
of which could be stopped and made to catch up again (a “chrono- 
graphe rattrapante”). Many readings were also made, in the course 
of the eclipse, with the thermopile screened; the zero proved very 
satisfactorily constant. 
Our second actinometrie set was especially intended to answer 
rapidly and to give a photographic record of the middie part of the 
radiation curve. It included a bolometer and a galvanometer with a 
moving coil of extremely small moment of inertia. Both instruments 
have been designed and constructed by Dr. W. J. H. Morr, who 
also was in charge of this equipment on eclipse day. The bolometer 
consisted of many strips of very thin platinum (Wollaston sheet) 
coated with lampblack, and mounted so as to form two equal gratings, 
one of which received the radiation. A thick copper frame warranted 
quick equalization of temperature of all screened parts, while an 
envelope of non-conducting material protected it against rapid external 
changes. The whole was fastened to the end of a tube with diaphragms, 
which was directed toward the sun by an assistant. 
As will appear from the photographie records, the galvanometer 
answered the purpose admirably (time of dead-beat swing less than 
one second; deflection 4 mm. for 1 microvolt; zero steady within 
0.1 millimeter); but the instrument being only a temporary one, 
adapted to the requirements of this eclipse and not yet to general 
use, Dr. Morr, who has since been improving the pattern, desires 
to publish full particulars at a later date. 
In order to obtain reasonable bridge-currents within the very wide 
range of sensitivity imposed by the phenomenon, the observer varied 
the resistance of the principal bolometer circuit by steps, as the eclipse 
proceeded, and each time read the strength of the main current on 
a milliammeter; the resistance in the bridge being left unaltered. 
That the zero reading of the sensitive galvanometer was very little 
influenced thereby, was a proof of the symmetry of the arrangement. 
s 
