464 THE LIFE OF JAMES D. FORCES. [CRAP. 



a satisfactory absolute determination of the thermal con- 

 ductivity of three different materials. In his reduction 

 of these experiments, Forbes unfortunately followed an 

 imperfect and inconvenient rule given by Poisson. Sir 

 W. Thomson has, however, applied the correct analysis, 

 and finds results not very different from those obtained 

 by Forbes. Another important feature in this inquiry 

 is that the series of thermometers buried in the trap-rock 

 of the Calton Hill, within the grounds of the Edinburgh 

 Observatory, are still (with the exception of the twelve- 

 foot one, which was broken by the intense cold of the 

 winter of 1860-1) read once a week, and have recently 

 furnished materials for a remarkable paper by the As- 

 tronomer Koyal for Scotland. 1 



We may next mention Forbes' elaborate paper, On the 

 Transparency of the Atmosphere, and the Law of Ex- 

 tinction of the Solar Rays in passing through it, based 

 chiefly on a valuable series of experiments made in the 

 Alps by means of the pyrheliometer ; which had the dis- 

 tinction of being made the Bakerian Lecture for 1842. 



In a paper entitled Hints towards a Classification 

 of Colours, read before the Royal Society of Edinburgh 

 December 4, 1848, and January 15, 1849, Forbes called 

 attention to the importance of a method of defining 

 colours with precision both for scientific and for artistic 

 purposes. In this paper he adopted from Lambert and 

 Mayer not only their arrangement of colours in a pyramid 

 or a triangle, but their choice of the colours which are to 

 be regarded as primary, namely red, yellow, and blue. 

 He afterwards attempted to form a permanent diagram of 

 colours selected from the collection of artificial enamels 

 employed in the Vatican fabric of mosaic pictures, by 

 comparing these enamels with the tints formed by the 

 mixture of the primaries on a rapidly revolving disk. 

 He found, however, on attempting to form a neutral gray, 

 by the combination of red, blue, and yellow, that the re- 

 sulting tint could not be rendered neutral by any com- 

 bination of these colours ; and the reason was found to 

 1 Edinb. Observations, 1860-70. Proc. R. S., 1871. 



