and the Blue Colour of the Sky. 429 



long, painted black inside, with one end covered with a cap 

 of thin black paper perforated with a hole 3 mm. in diameter. 

 If the end of the tube is pressed into the socket of the eye 

 and the tube directed towards some object, the blackness of 

 which we wish to test, the hole is visible as a more or less 

 feebly illuminated disk, unless the object is absolutely black, 

 in which case the hole is invisible. It is advisable to have 

 an extension tube projecting beyond the cap, otherwise the 

 edge of the hole may be seen illuminated by diffracted light. 

 This device is so sensitive that it will show the scattering of 

 light by one foot of air on a clear day, illuminated by normal 

 sunlight : in fact, I have detected the scattering of 1 foot of 

 air illuminated by the light of the blue sky ten minutes 

 before sunset. As a background I employed a black cave, a 

 wooden box 1*5 X 1 x 1 metres, with a hole 40 cm. square at 

 the end, painted black inside and furnished with a curtain 

 of black velvet at the back. This box is placed out of doors 

 in the sunshine, the sun's rays being parallel to the ends. If 

 the nigrometer is brought close up to the aperture, the hole 

 at the end of the tube disappears, coming into view as a very 

 pale bluish disk when we move back about 30 cm. If we 

 look at the open window of a house 1000 feet away on a clear 

 day it appears extremely black, and if we imagine this to be 

 reduced to 1/1000 of its intensity we should expect that the 

 residual light would be unappreciable by the eye, yet this is 

 what we actually see with our black cave at a distance of 

 one foot. 



Coming back now to our experiment with the long tube, we 

 find that if we observe the interior with the nigrometer the 

 scattered light is very conspicuous. If, now, the scattered 

 intensity in directions nearly parallel to the exciting beam 

 and in directions at right angles to it are as 2 to 1, the long 

 tube should show an intensity twice as great as a stratum of 

 air 3 metres in depth, traversed by sunlight in a direction 

 perpendicular to the line of vision. This comparison was 

 made in the following way on a very clear day. The dark 

 cave previously referred to, was placed at a distance of 

 3 metres from the open door a little to one side of the 

 heliostat. By means of a mirror and a silvered sliver of glass 

 placed in front of the observation hole of the long tube, a 

 photometric comparison was made without difficulty. All 

 windows in the laboratory were darkened, and screens of 

 black cardboard were mounted as required to exclude un- 

 necessary light ; the aperture of the door was also contracted 

 with curtains and screens. It was found that the luminosity 

 in front of the dark cave was at least twice as bright as that 



