36 SMITHSONIAN MISCELLANI^OUS COLLECTIONS VOL. 52 



By interposing" this absorbing screen the proportion of the 

 observed radiation which lay in the infra-red spectrum could be 

 roughly determined. Various trials made on Flint Island showed 

 that ordinary sun-rays comprised from 29 to 37 per cent of rays 

 transmissible by this screen, depending on the humidity of the air 

 and the altitude of the sun ; whereas sky-rays were only about 

 20 to 25 per cent transmissible. Moon-brightness (and by this is 

 meant reflected sun-rays, not ra)s proper to the moon itself, for 

 such were eliminated by the glass plate) was examined on one 

 occasion and showed a transmissibility of about 50 per cent. 



Several diaphragms were provided for graduating the aperture 

 of the concave mirror. The apertures of these diaphragms were 

 knife-edged, and those of less than i centimeter diameter were 

 adjusted to lie within 3 millimeters of the silvered surface of the 

 mirror. Allowing for the portions of the mirror shaded by the 

 bolometer and its adjuncts, the apertures available were as follows: 



Area (sq. cm. 

 Factor 



i,6;8.o 



283.0 62.0 



0.1750 



0.0383 



0.316 I 0.077 

 0.000195 I o.oooo^l 



The equatorial was set up at Flint Island, on the beach, at about 

 12 meters' distance from the galvanometers used for observing the 

 indications of the bolometer. Two galvanometers were provided, 

 exactly alike in resistance and general construction, and arranged so 

 that if at the last moment any accident should happen to one, the 

 observer might pass at once to the other.^ A thatched hut shaded by 

 palm trees sheltered the galvanometers and their appliances and 

 was found to give most satisfactory protection both from heat and 

 rain. The galvanometers were each of 1.5 ohms total resistance, 

 composed of 12 coils all connected in series. The needle systems, 

 of 30 needles each, had mirrors i mm. by 1.2 mm. and weighed com- 

 plete o.oi I gram each. Acetylene lamps were employed as light 

 sources, and the images of the narrow flames were read on ground- 

 glass scales 90 centimeters in front of the galvanometers. Resistances 

 of 3, 8, 17, 45, 200, and 1,000 ohms, respectively, could be put in 

 series with either galvanometer to reduce its deflections if required. 

 The corresponding factors of reduction are 2.0, 4.0, 6.0. 13.1, 60.0, 

 and 300. These numbers were obtained by actual trial. 



The Wheatstone's bridge of the bolometer comprised the twO' 

 platinum strips of 0.5 ohm each and two coils of 5.0 ohms each. 



^ This prudent measure was suggested by Mrs. Abbot. 



