122 Mr. Loyd A. Jones on a Method and Instrument 



This wedge varies continually in opacity from one end to the 4 

 other, and is so arranged as to move across the axis of sight* 

 The object lens, L^ is of such focal length that the object of 

 which the visibility is to be measured is imaged sharply at, 



Fij 



The Visibility Meter Open. 

 Fijr. 9. 



The Visibility-Meter Closed. 



the point where the reflecting surface, M, intersects the axis 

 of sight. This lens is adjustable in position, so that either 

 near or far objects maybe imaged in the proper plane. The 

 eye lens, L 2 , is of such power that the magnification of the 

 system is unity, thus giving a retinal image of the same size 

 as when the object is viewed with the naked eye. The lens 

 L 2 enables the eye to see clearly defined the image formed 

 by the lens L l5 The mirror M may be replaced by a simple 

 photometric field for the purpose of measuring brightness, it 



