48 Dr. E. S. Clay. On the Application of 



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 



