M. H. Helmboltz on the Theonj of Compound Colours. 525 



dispatch, and to show them to others, I place at the above 

 distance from the eye-glass a dark moveable screen, with a small 

 round orifice, through which the observer looks towards the eye- 

 glass of the instrument. If it be wished, instead of the com- 

 pound colour, to see the two composing simple ones, one slit 

 after the other may be closed by a second person, so that only 

 one of the two mixed colours remains standing; or a second 

 small prism is introduced between the eye and the orifice ; and 

 thus, instead of a single bright spot in the eye-glass of the tele- 

 cope, two with distinct colours are observed. For the more 

 certain determination of mixed colours which approach very 

 nearly to white, it is useful to encircle the opening of the eye- 

 piece with a sheet of white paper illuminated by white light, 

 and to compare its colour with the observed one. I have also 

 noticed that the eye is rendered less sensitive to fine diff(;rences 

 of coloui- by long gazing upon very whitish mixed colours; and 

 it is therefore advisable to permit th« eye to rest at times, or to 

 allow it to wander over the suiTounding objects. When the ob- 

 servation is renewed, a mixture of colour is often plainly detected 

 in the apparent white which had previously escaped observation, 

 and which, when long looked at, again disappears. 



In this way it is possible to obtain the total combination of 

 every two of the simple prismatic rays in all degrees of relative 

 strength, and to observe them undisturbed by the presence of 

 other colours. My observations, the principal points of which I 

 have had corroborated by the testimony of several other persons 

 practised in the judgement of colours, thus avoiding whatever 

 error the subjective defects of my own eyes might occasion, have 

 furnished the following results, some of which differ, in a sur- 

 prising manner, from the views on this subject heretofore held. 



1. Red gives with orange a redder orange; with yellow, 

 orange : the mixed colours do not differ sensibly frona the de- 

 grees of orange which appear in the simple spectrum. With 

 green it gives a yellow, which, less saturated, is paler than the 

 simple yellow, and which, when red is predominant, passes 

 through orange into red, and when green is predominant, passes 

 through yellow-green into green. With the grccn-blue tones of 

 the spectrum, a fiesh-colour is obtained; with tlic sky-blue ones 

 a rose-red colour, wliich, when blue predominates, j)asses into 

 whitish violet, but when red predominates, passes into carmine- 

 red. When, finally, the red is combined with the indigo and 

 violet rays, which lie further towards the end of the spectrum, a 

 purple-red of increasing depth and saturation is obtained. 



2. Orange gives with yellow a yellower orange ; with green, a 

 pale yellow; with blue, flesh-coloured tones, which, with indigo 

 and violet, pass over into carmine-red. 



