Page:Proceedings of the Royal Society of London Vol 69.djvu/65

Rh As the worst ink was always the Blue, I attempted to find a colour which should be more transparent in the green. I obtained a bright green rubbing from Fleming, but I found it was green not because it was more transparent in the green and yellow-green, but because it was more opaque in the blue and violet. There seems to be no ink in the market at present that is sufficiently transparent in the yellow- green.

The intensity of the reflected beam is given (for any one colour) by I' = I a e, where e is the depth and I is the original intensity of the beam. For another depth of tint /

I" = la', = eloga, =/logff,

K ,1.1

.-., = log f -hlog r.

J 1

Thus the ratio of the log ought to be a constant through the spectrum. Neglecting the readings in which the aperture was too small to be taken accurately (owing to the backlash of the sector), and in which also the scattered light is of importance, the following results show a very close agreement with theory.

The inks were only rubbed on with the finger, so that the patches were not exactly uniform, especially the lightest one (1). In this case

Curves showing the absorptions of the same ink printed to different depths.

the readings were checked by drawing a smooth curve between the points found, and the numbers taken from the curve, where they were