for the Measurement of Visibility of Objects. 



127 



])lane so that its brightness may be read from the designated 

 point o£ observation. 



The procedure in taking a complete visibility reading then 

 consists of three steps : — 



(1) With the glare field in position the instrument is so 

 set that the image of the object occupies a position in the 

 field X, fig. 7, such that the dividing line between a and b 

 approximately bisects the image. The hand wheel N is then 

 turned until the object is just visible in field 6, and is in- 

 visible in field a. The position of the index on the scale S 

 is then read giving the scale reading Sj. 



(2) The photometric field is thrown into position and the 

 instrument so aligned that the image of the background fills 

 the portion d of the field Y, fig. 7. N is then turned until 

 d and c are equal in brightness and the position of the index 

 being read gives the scale reading S 2 . 



(3) The alignment of the instrument is changed so that 

 the image of the test plane fills the field d and a photometric 

 balance is again made by turning N. The position of the 



index now gives the third scale r< 



Fig-. 10. 



ading S 3 . 



CALIBRATION • CUfc-VE ■ A 

 VEILING GLARE, Bv, vs SCALE READING 5, 























\1 







1 





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! ; 1 

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BO 

 .60 

 .40 

 -.20 

 3.00 







l\ 



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i 









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A 



Ns 





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' [ , 















































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 1 1' 







j 



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r 







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£0 

















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k 









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.20 





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1 



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v 1 1 







5CALE (UAOINC 

 Calibration Curve A. 



Two calibration curves for the instrument must be pre- 

 pared, shown in figs. 10, 11, and 12. Curve A is for veiling 



