luminosity curve of the normal spectrum. It is obvious from them that inks whose absorptions terminate at X 49 and A 59 will be better than those terminating at the points of intersection of the ray com- position curves.
PART III. EXPERIMENTAL.
15. The Apparatus.
Throughout the following experiments we used a copy of Abney's colour-patch apparatus. The only difference in the main instrument was the use of a right-angle prism in front of the collimator slit to reflect the light from the lantern down the tube. This enabled us to set the apparatus up much more nearly in a direct line, and therefore to arrange it all on an ordinary long table.
The collimator was about 13 inches long, with a 2^-inch achromatic lens. The slit was one of Elliott Brothers'.
The prisms were equilateral prisms cut from a very fine specimen of white glass especially selected for this purpose. The face of each was about 2f by 2-| inches.
The projecting lens was an achromatic one, 2^ inches diameter and about 30 inches focus. It produced a visible spectrum 8 cms. long, which was very bright and sharp.
The slit was mounted in a brass frame arranged to slide horizontally across the spectrum in its focal plane. It was about 1\ inches high, and usually about '5 mm. open. A small 1-inch right-angle prism, carried by the same frame, was arranged to reflect the light from the top half of the slit to one side on to a second and larger prism, also carried on this frame, which reflected it forward.
Combining Lenses. These were two 6-inch lenses of about 20 inches focus, one of which received the light from the lower half of the slit and focussed it on the patch. This is the direct beam. The other lens focussed the reflected beam on the same patch. This patch is the image of the aperture of the double-image prism mentioned below. The distances of the lenses were adjusted until the horizontal edges of the patch were sharp, and were then rotated slightly round a vertical axis until the patch remained stationary as the slit was moved across the spectrum.
The patches were formed by cutting a small square window in a piece of black velvet pasted on a card. To avoid stray light the card was placed at the far end of a box, 2 feet long, blacked all over. The mouth of the box was covered with brown paper, and holes were cut in this just large enough to admit the two beams. A part of one side Avas removed to enable the patch to be observed. A blackened pillar was placed, as in the Rumford photometer, to obtain a sharp dividing line, one-half of the patch being lighted by
