ON THE THEORY OF COMPOUND COLOURS. 



VII. Method of Observation. 



The instrument is turned with the end AB towards a board, covered with 

 white paper, and illuminated by sunlight. The operator sits at the end AB, to 

 move the sliders, and adjust the slits ; and the observer sits at the end E, 

 which is shaded from any bright light. The operator then places the slits so 

 that their centres correspond to the three standard colours, and adjusts their 

 breadths till the observer sees the prism illuminated with pure white light of 

 the same intensity with that reflected by the mirror M. In order to do this, 

 the observer must tell the operator what difference he observes in the two halves 

 of the illuminated field, and the operator must alter the breadth of the slits 

 accordingly, always keeping the centre of each slit at the proper point of the 

 scale. The observer may call for more or less red, blue or green ; and then 

 the operator must increase or diminish the width of the slits X, Y, and Z 

 respectively. If the variable field is darker or lighter than the constant field, 

 the operator must widen or narrow all the slits in the same proportion. When 

 the variable part of the field is nearly adjusted, it often happens that the 

 constant white light from the mirror appears tinged with the complementary 

 colour. This is an indication of what is required to make the resemblance of 

 the two parts of the field of view perfect. When no difference can be detected 

 between the two parts of the field, either in colour or in brightness, the observer 

 must look away for some time, to relieve the strain on the eye, and then look 

 again. If the eye thus refreshed still judges the two parts of the field to be 

 equal, the observation may be considered complete, and the operator must measure 

 the breadth of each slit by means of the wedge, as before described, and write 

 down the result as a colour-equation, thus 



Oct. 18, J. 18'5(24) + 27(44) + 37(68) = W* (13). 



This equation means that on the 18th of October the observer J. (myself) made 

 an observation in which the breadth of the slit X was 18 '5, as measured by 

 the wedge, while its centre was at the division (24) of the scale ; that the breadths 

 of }' and Z were 27 and 37, and their positions (44) and (68) ; and that the 

 illumination produced by these slits was exactly equal, in my estimation as an 

 observer, to the constant white W. 



