MaxnoeWs Curves to Three-colour WorJc, &c. 49 



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 



VOL. LXIX. j; 



