the direct beam and the other half by the reflected beam. The two patches were adjusted (before the pillar was inserted) to exactly coincide, so that any want of uniformity in the illumination might affect them equally ; this we found most important. The patches were slightly larger than the window, so that it was lighted up to its edges. The pigment to be compared was always placed in the reflected beam, the white beam falling on a piece of the same paper that the pigment had been printed upon.
The sector was one made by Hilger upon the lines laid down by Abney. It closes entirely, and opens to one-half of the full beam. The aper- ture is divided into 100 parts, so that percentage aperture can be read directly. It has to run at a high speed to obtain good results. I found that there was a lot of backlash in the sleeve, amounting to about two divisions. To avoid the error due to this, it must be stopped always in the same way. I found by exactly closing the sector while it is running which can be done very accurately by looking through it to the light and adjusting it until the light is entirely extinguished and then stopping it by the sleeve, the reading is nearly always zero. If in addition the handle has last been moved in the direction to dose the aperture, the reading was always found to be correct. These experiments took a long time. The sector was placed in the direct beam just in front of the box spoken of above.
As the direct beam is cut down to one-half by the sector, even when the latter is fully open, it is necessary to reduce the reflected beam also to enable a balance to be obtained. This was done by attaching a Nicol's prism to the frame carrying the slit and the two right-angled prisms in the beam between the two latter. As the light has already been polarised by the double-image prism, it is possible by rotating this Nicol to reduce its luminosity to any desired extent.
The double-image prism was about 1 inch square aperture. It was at first mounted against the collimator lens, but finally was placed against the projecting lens. Its adjustment is of very great import- ance, and it was found impossible to set it with sufficient accuracy by hand. It was therefore mounted in a brass frame which could be given a very slow rotation by means of a screw, and after the prism had been adjusted as nearly as possible by hand, it was finally corrected by this screw. Its rotation causes the two spectra, into which it divides the original one, to move relatively across one another, and therefore varies the colour which passes through the slit. The prism is rotated until, with the slit in the yellow, the two beams are exactly the same colour. As the slightest alteration here is very easily perceived, the adjustment can be effected very accurately, and will then be correct throughout the spectrum.
A glass Zeiss millimetre scale was temporarily attached to the top of the camera, and a pointer on the frame carrying the slit moved
