for the Measurement of Visibility of Objects. 



121 



The letter M, fig. 7, indicates a semi-transparent mirror set 

 at 45° to the axis of the instrument, which is coincident with 

 the axis of sight. This mirror reflects about 50 per cent, 

 of the incident light and transmits approximately 20 per 

 cent., the remaining 20 per cent, being absorbed. The 



Fi*. 7. 



iScale T 



ii:|M!;.^il|6l|l|7|llHI^!I^JiP'p|lffi 



•Secondary Ax Is , = — ^~ 



Diagrammatic Illustration of the Visibility Meter. 



secondary axis of the instrument is a line perpendicular to 

 the axis of sight at the point where the reflecting surface 

 intersects the axis of sight. C is a diffusing member com- 

 posed of pot or flashed opal glass placed perpendicular to the 

 secondary axis. This diffusing surface is illuminated by a 

 light source, S, mounted so as to move along the secondary 

 axis. This source is mounted at one end of a brass tube, B, 

 -on one side of which is the rack, R^ A knurled hand wheel, N, 

 is mounted rigidly on a shaft carrying also the pinion P l5 

 which engages the rack R 1} thus providing for the movement 

 of the source along the secondary axis by a rotation of the 

 hand wheel, N. A second pinion, P 2 , in mesh with P 1? 

 engages the rack R 2 , which is rigidly attached to the frame 

 •carrying the neutral gray non-diffusing optical wedge, W. 



