for the Measurement of Visibility of Objects. 123 



being necessary to measure B x in order to obtain the visibility 

 value. The details of the interchangeable Held are shown in 

 the small right hand drawing of! fig. 7. 



In order to increase the sensibility of the instrument in 

 the measurement of B r , a strip of clear gelatine film is placed 

 over one-half of the field, thus increasing the reflecting power 

 of that half by about 8 per cent. If a be the half of the field 

 thus covered, it will for a given position of the source, S T 

 cause a brighter veiling glare to appear as superposed over 

 the object and background than the other half of the field, Ik 

 Now, if the object be so adjusted in the field of the instru- 

 ment that the line between the two halves of the field bisects 

 (approximately) the object, that part of the object in the 

 field b will still be visible when the other part (that in field a) 

 has been completely obscured by the veiling glare of that 

 part of the field. In this way certain limits are set between 

 which every setting must be made. If it is desired to de- 

 crease the difference in brightness between the two parts of 

 the field (in order to narrow the limits of the setting), this- 

 may be done by making the reflecting powers of one-half of 

 the field any desired amount greater than the other. This 

 is easily accomplished if the reflecting surface is made by 

 depositing the reflecting metal on the glass by a cathodic 

 discharge. 



The photometric field is made by covering one-half, c, of 

 the small circular opening with a piece of brightly reflecting 

 matt paper. The white paper is illuminated by the source, S> 

 and the other half, d, being open, permits the eye to see the- 

 image of the object or background formed by the lens 1^. 

 The line between c and d (when the photometric field is in a 

 position for use) lies at the intersection of the axis of sight 

 with the secondary axis and hence is the plane in which the 

 object is imaged by the lens L 2 . The circular field e sur- 

 rounding the photometric fields, c and d, is made of grey 

 matt paper. This is the type of field actually used in the- 

 instrument, but any of the well-known types, such as a 

 Lummer-Brodhun cube, may be substituted if desired, the- 

 chief requirement being a sharp dividing line lying in the 

 image plane. These two fields are mounted in a small metal 

 frame sliding in milled metal ways placed perpendicular to 

 the plane through the axis of the instrument. Stops are 

 provided so that each field may easily and quickly be 

 brought into position with the axis of sight passing through 

 the centre of the field being used. A light filter is placed 

 at F. This filter is of such quality that the light from the 

 source S after passing through the filter matches in colour 



