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



the light which illuminates the object and background. A 

 graduated scale, T, is fastened rigidly to the case which 

 encloses the source. An index, I, attached to the source or 

 its supporting member moves along this scale indicating at 

 all times the position of the source, S, and wedge, W, with 

 reference to the axis of sight. The scale is so calibrated that 

 from the position of the index at any instant the brightness 

 of the glare field B 1 and the transmission of the wedge (T ) 

 on the axis of sight can be determined. By turning the 

 hand wheel N the source S and the wedge W are caused to 

 move simultaneously and in such fashion that an increase in 

 the brightness of the veiling glare, B x , is accompanied by a 

 decrease in the transmission of the wedge of the axis of sight. 

 This decrease in transmission causes a diminution in the 

 intensity of the light which reaches the eye from the object 

 and background. 



Now the light transmitted by the diffusing member C is 

 reflected into the eye, appearing to come from the image of 

 that surface. Thus diffuse white light is caused to enter the 

 eye from a point between the eye and the object being ob- 

 served, increasing the apparent brightness of both object and 

 background by the same amount and causing the ratio of 

 Bx to B 2 to be lowered. At the same time the wedge W is 

 introduced causing a proportionate reduction in the apparent 

 brightness of both B x and B 2 . 



It will be seen, therefore, that the action of the instrument 

 is exactly analogous to the action of material particles distri- 

 buted through the foreground space, i. <?., the production of 

 a veiling glare between the eye and the object and the ab- 

 sorption of a certain percentage of the light reflected or 

 emitted by the object and background. Now, if the source 

 and wedge be moved to such a position that the object is 

 just visible in one part of the field and not visible in the 

 other a setting is obtained from which the values of B r and 

 T a can be determined. 



In our fundamental equations, -B was defined as the bright- 

 ness of the veiling glare which when superposed over object 

 and background will reduce the visibility to zero or a just 

 perceptible value. It will be noted that as no term covering 

 the decrease in brightness due to absorption of light in the 

 foreground space appears in those equations, it is inferred 

 that the entire loss of visibility is produced by the veiling 

 glare, B«. Now, the effect of the introduction of an ab- 

 sorbing member such as the wedge, W, is merely to decrease 

 the amount of veiling glare required to reduce the visibility 

 to zero. It is not feasible in practice to produce the extinction 



