Fig. 26.—Absorption of transmitted and reflected Light by Prussian Blue and Carmine.
On page 108 we have given the diagram of the luminosity of the spectrum in reference to a standard white light. It will bring this luminosity more home if, in a similar manner to that described above, we make a template of this curve ([Fig. 25]). We can place a narrow slit horizontally in front of the condensing lens of the optical lantern, and throw an image of it on to the screen. If in close contact with this slit we rotate the template, we shall have on the screen a graduated strip of white light, giving in black and white the apparent luminosity of the spectrum as seen by the eye.
Fig. 25.—Template of Luminosity of White Light.
It has been stated in chapter V., that it is generally immaterial whether a pigment is in contact with the paper or away from it, so long as the light passes through the pigment. The above figure ([Fig. 26]) shows the truth of this assertion. I. and II. are the curves taken of the light transmitted by Prussian blue and carmine respectively, and III. and IV., from the light reflected from these colours on paper.
Fig. 27.—Collimator for comparing the intensity of two sources of Light.