254 Prof. Grassmann on the Theory of Compound Colours. 



The heat transformed into power, as given in the fifth column, 

 has been reduced to Centigrade degrees in liquid water, by di- 

 viding the duty of a pound of steam by Mr. Joule's equivalent, 

 1389'6 feet per Centigrade degree. 



In these examples, founded on the calculated and obsei'ved 

 duty of Mr. Wicksteed's engine during experiment F, the actual 

 ratio is less than half the maximum. This waste of heat is to 

 be ascribed to the following causes : — 



First. The expenditure of heat in raising the feed-water to 

 the temperature of evaporation. 



Secondly. The initial pressure in the cylinder is but 18'931bs. 

 on the square inch, while that in the boiler is 45*7. 



Thirdly. The expansive working of the steam, instead of being 

 continued down to 30° Centigrade, the temperature of the con- 

 denser, stops at a much higher temperature, 74°'66. This is 

 the most important cause of loss of power. 



XXXVII. On the Theory of Compound Colours. 

 By Professor Grassmann*. 



[With a Plate.] 



IN the 87th volume of this journal f, Helmholtz published a 

 series of observations, some of which were new and inge-. 

 nious, and from which he comes to the conclusion, that the 

 theory of compound colours, universally admitted since the time 

 of Newton, is erroneous in its most essential points ; and that there 

 are properly only two prismatic colours, yellow and indigo, which 

 when mixed give white. It may consequently not be regarded 

 as superfluous to show that the Newtonian theory of compound 

 colours is correct to a certain point, and especially that the 

 opinion that every colour has its complementary colour, with 

 which when mixed it gives white, is founded upon mathemati- 

 cally iucontcstable facts, so that this opinion must be regarded 

 as one of the most tenable in ])hysics. I will then show that 

 \h.<i positive observations of Helmholtz, instead of tending to the 

 subversion of this theory, serve rather partly to prove and partly 

 to complete it. 



For this purpose it will be necessary to analyse the impression 

 of colour of which the eye is capable into its elements. The 

 eye first of all distinguishes light as coloured and colourless. In 

 colourless light (white, gray) it only distinguishes a greater or 

 less intensity, and this may be mathematically determined. 



* From the AnvaJ.en der Pliysik und Chemie, 1853, p. 69. 

 t Philosophical Magazine, S. 4. vol. iv, p. 519. 



