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



this space behind the object plane may either raise or lower 

 the visibility of the object, depending on the precise existing: 

 conditions. Hence the presence of such material cannot in 

 general be termed a cause of lowered visibility, although 

 in some cases it may operate in that direction. The bright- 

 ness B x is in this case also a result of the summation of 

 the light entering the eye from any and all points within the 

 angle g>, and from between A and infinity. This light is 

 scattered sunlight, the scattering being due to diffraction,, 

 reflexion, and refraction, the last two factors being pre- 

 dominant in the case of cloudy sky or of mist in the 

 atmosphere. 



Now let us consider the case where scattering material is 

 found also in the space between the eye, A, and the object 

 plane. This material may be localized in a particular region 

 or uniformly distributed throughout the space from A to the 

 object plane. The presence of this finely divided material, 

 such as dust, mist, &c, will operate in two ways upon the 

 apparent values of the brightness of object and background. 

 A decrease in the intensity of the light emanating from the 

 object and background space will be accompanied by an 

 increase in the intensity of the light apparently emitted by 

 the foreground space. That is, T d and F d in general vary 

 according to some inverse law. In order to separate the 

 action of these factors let us take it that T d = unity and allow 

 Prf to vary. Some value of P^ is thus added to both B 1 and 

 B 2 , and it will be seen that whatever the value of P^ the 

 ratio of B x to B 2 will be decreased, thus causing a lowering 

 of visibility. Assuming a small constant value of P^, it will 

 be seen that an increase in T d will change both B 1 and B 2 in 

 the same proportion, thus keeping their ratio constant. A 

 variation in T^, therefore, affects the visibility only in so far 

 as the change in B x governs the value of k (the contrast 

 factor of the eye). Since the highest values of B t found in 

 nature are not above the point where 



AB 



b; 



begins to increase (due to very hii>h values of BJ, it follows 

 that if a variation in T d has any effect upon visibility it will 

 be to lower the value of that term. 



As stated previously, the relation between the terms T 

 (transmission) and P (effective brightness) is in general 

 expressed by some inverse function. Thus as P increases 

 in magnitude, T usually diminishes. No general expression 



