NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 5 



ment was taken to Washington the latter part of December to have 

 an overhauHng", and after that the constant was either' 2. ii or 2.12. 

 But it plainly shows that the constant does not vary appreciably, if 

 the instrument has reasonable care. The slight change in Decem- 

 ber was probably caused by the blackening of the surfaces of the 

 manganin strips being altered a very little. 



SENSITIVENESS OF PYRANOMETER 

 In March, 1918, at the Hump Mountain Observatory an investi- 

 gation was made of the sensitiveness of the pyranometer to dififerent 

 wave lengths. To do this, the regular bolometric apparatus was 

 employed, except that in place of the bolometer and battery, the 

 pyranometer was used to measure the heat of the various parts of 

 the spectrum, and its indications were recorded as on a regular 

 holographic plate. The thermo-elements were joined directly to the 

 very sensitive galvanometer, used in holographic work. (As a 

 matter of precaution shunts were first put in the circuit to prevent 

 any possible injury to the galvanometer.) A circular piece of black 

 paper with a slit 0.40 mm. wide, and cut just to fit within the glass 

 hemisphere, was placed over the blackened strips and at right angles 

 to them. Then the glass hemisphere was placed in position. Out- 

 side of this was placed a black metal diaphragm, and finally the 

 sun hood mentioned above under (4). Thus there was practically 

 no chance for stray radiation to reach the strips, especially since 

 all the apparatus was installed in a dark room with black paper lined 

 walls. 



A brief description of the remainder of the spectrobolometric 

 apparatus employed for this test will be given. The sun's rays 

 strike a two-mirror coelostat and are reflected into the build- 

 ing, first passing through the revolving sector to cut down the 

 intensity of radiation in a fixed and known ratio. Then they pass 

 through the slit, and about eight feet distant, through the prism. 

 The spectrum is reflected by a plane speculum metal mirror on to 

 a concave speculum mirror, which brings the spectrum to a focus 

 at the plane of the pyranometer strips. (The strips lie in a vertical 

 plane in this observation, with their longitudinal direction horizontal, 

 and crossed at the center by the paper slit, which allows only rays 

 of nearly the same wave length to strike the strips at one time.) 

 The spectrum is moved across the paper slit, when the prism is 

 rotated by a worm gearing. This gearing is connected to the plate 

 carrier clock in such a manner that as the photographic plate descends 



