JBrtabtnegg of flM&niflbt Sfti? 163 



really reliable test is one carried out in a properly 

 situated workshop where equilibrium of temperature 

 can be maintained." 



BRIGHTNESS OF MIDNIGHT SKY 



Some interesting observations made by Mr. Gavin 

 J. Burns, at Blackheath, have led him to the con- 

 clusion that the brightness of the midnight sky is in 

 ordinary circumstances a constant quantity. He fixed 

 upon the northern sky, near the North Pole, for his 

 tests, and though there were variations in the 

 luminosity, he believes that they can be explained 

 without assuming any variation in the intrinsic 

 brightness of the sky itself. A high degree of 

 luminosity on August llth, 1913, he thinks, was most 

 likely due to a slight amount of twilight, whilst a 

 low degree on February 22nd and 27th, 1914, was 

 doubtless to be attributed to an absence of small 

 particles in the atmosphere, and a consequent lowering 

 of the reflective power. A low degree, also, on 

 March 18th and 19th, 1914, was probably owing to an 

 absence of dust caused by heavy rain during the day. 



When Mr. Burns' fellow-members of the British 

 Astronomical Association discussed his experiments, 

 the question was asked by Mr. H. J. Bunker whether 

 the least brightness would not occur somewhere near 

 the time of the new moon. Mr. Burns' observations, 

 however, were made when the moon was below the 

 horizon, and his impression was that if any moonlight 

 was visible the amount would be quite insignificant. 

 One or two of the observations were made on clear 

 nights, though cloudy. There were breaks in the 



