428 Prof. K. W. Wood on Light Scattering by Air 



long and 12 cm. in diameter, painted black inside. The 

 ends were closed with wooden caps lined with black velvet; 

 they are shown in section in fig. 1, The front cap was per- 

 forated by a hole 3 cm. in diameter near the edge of the tube, 



Fig. 1. 



% VELVET 

 4 



I LENS ".I 



and a lens of 3 metres focus cemented over it. This lens 

 forms an image of the sun at the opposite end of the tube of 

 the same diameter as its clear aperture, consequently the 

 tube is traversed by a cylindrical beam of sunlight of constant 

 cross-section with sharp edges, The cap at the back of the 

 tube was also perforated with a hole, which was covered with 

 paper painted black with the exception of a narrow strip 

 near the edge of the hole. A hole 3 mm. in diameter was 

 burnt through the centre of the paper disk with a hot glass 

 tube. A heliostat was mounted outside of the door of the 

 laboratory and the sunlight reflected from a small mirror 

 through the long-focus lens down the tube. By observing 

 the unpainted portion of the paper disk the edge of the solar 

 image was brought up to within about 5 mm. of the smull 

 aperture, through which the observations were made. A 

 vertical diaphragm (A) concealed the lens and shut off the 

 glare diffracted by its edges and small scratches on its 

 surface. It was thus possible to bring the pupil of the 

 eye up to within 5 mm. of the cylindrical beam of sunlight, 

 and look diagonally across it down the tube at the black 

 velvet background, the direction of vision making an angle 

 of less than one degree with the light-rays. Before the 

 introduction of filtered air the amount of light scattered 

 along the path of the beam was considerable, each mote 

 shining with dazzling brilliancy. With filtered air the 

 velvet background appeared quite black, but the residual 

 luminosity of the intervening air was at once apparent when 

 examined with a simple piece of apparatus, which, for 

 want of a better name, we may call a nigrometer. It is 

 merely a tube of pasteboard 3 cm. in diameter and 40 cm. 



