198 Orthochromatic Photography. [Sess. 
coated : this is the method usually followed by manufacturers. 
The second way of applying the dye is by soaking ordinary 
dry plates in the dye solution with certain precautions. This 
process may be practised with success by amateurs, but I do 
not purpose going into it to-night. This diagram (fig. 3) 
shows by curves the effect of two of these dyes on the colour 
sensitiveness of a plate—a, as exhibited on an ordinary plate ; 
Ais, BG, 58.) Evi Cc Ch bale 
Fig. 3.—Zffect of Dyes in Photography. 
b, the effect of cyanin; and c, the increase of sensitiveness 
due to the use of erythrosin. I will now show you a series 
of photographs of the spectrum, taken by myself on various 
brands of orthochromatic plates without the intervention of 
any light-filter to modify their actual sensitiveness. The 
spectrum employed was that derived from a Welsbach gas- 
light C burner. The light was passed through a small direct- 
vision spectroscope, kindly lent to me by my friend Mr 
Stenhouse. The spectroscope was fixed in front of the 
camera, taking the place of the lens, the spectrum being 
focussed on the screen and means taken that every plate 
should register exactly in the same position. Foundation 
datum marks were obtained from the sodium line D, and the 
potassium line K,, which was got by photography. The 
other lines were then obtained by comparison with a spectrum 
chart. I thought it best to take the Frauenhofer lines in this 
way rather than to attempt colouring the slides. The plates 
tested were—Ilford chromatic (Plate XXIII. 3), Edwards’ 
isochromatic (Plate XXIII. 4), Lumiére A (Plate XXIII. 5), 
Lumitre B (Plate XXIII. 6), Lumiére C (Plate XXIII. 7), 
and Cadett spectrum (Plate XXIII. 8). You will notice the 
tendency to an insensitive gap in these plates between the 
