Gradation of a Developed Photographic Image. 



303 



Fig. 1. 



described in previous papers on Colour Photometry in the ' Philo- 

 sophical Transactions,' was brought into use. A ray of the spectrum 

 was allowed to issue through S, fig. 2, and after passing through 



a lens formed a square patch of monochromatic light on C, a 

 white screen. In the path of the beam X a plain glass mirror, Mi, 

 was inserted, which deflected a certain percentage of the beam Y 

 to Mjj, a silvered glass mirror, which in its turn reflected Y so as to 

 fall on C. A rod, R, placed in proper position, caused two oblongs of 

 the direct and reflected beams to fall side by side on C. Two sectors, 

 A and B, were placed in the paths of X and Y respectively. The 

 apertures of A could be opened or closed at pleasure whilst the disc 

 was rotating. A red ray of the spectrum first came through S, and 

 the aperture in A required to equalise the two adjacent patches of 

 light was noted. Other rays of the spectrum were similarly dealt with, 

 when it was found that the aperture in A remained unaltered, showing 

 that within the limits of error of observation the percentage of reflec- 



Fig. 2. 



