260 



Captain Abney and Major-General Festing. [Dec. 8, 



dimensions) was constructed. Take Nos. 1 and 2, Table VII, the 

 exposures given to each were the same, viz., 15 seconds. The light 

 transmitted through each square was read off, and found to be identical, 

 viz., 21 '5. In the scale diagram the abscissa of the curve having the 

 ordinate 21*5 is 66, which is the equivalent time exposure, which is 

 also a measure of the intensity. If the exposure had been prolonged 

 to 1 minute it would be equivalent to an intensity of 264 (since the 

 measured exposure was only 15 seconds). The other intensities were 

 calculated in a similar manner. The intensities 21 and 264 were 

 taken as points on the parabola, and the intermediate intensities 

 calculated using the formula W—m = nVy. 



Fig. 6. 

 Time Scale Diagram. 



JNT.B. — It will be noticed that in the above diagram the abscissae are marked as 

 square or relative exposure. A reference to the time scale table will show that the 

 exposure = number of square K 5. Eor simplicity's Sake the; number of the square 

 bas been taken as the unit of time. ■ • * < - • - • 



The following is another example of the photographic measure- 

 ment. . .. - . .. • . 



