NATURAL PHILOSOPHY. 179 



(centigrade), while it is fertile in Dalmatia, where the temperature 

 averages only 13; the limit of grape vines ends upon the banks of 

 the Loire, while it includes, on the banks of the Khine, regions where 

 the average temperature never exceeds 10. Harvest is secured in 

 London with a temperature of 17, while at the same time it is made 

 at Upsal, where the average temperature is only 15. We are thus 

 forced to acknowledge that these phenomena depend on the presence 

 of an important element of calorification : luminous heat, which ele- 

 vates the temperature of opaque bodies above that they can receive 

 from the diffused heat of the atmosphere. 



M. de Gasparin has adopted an instrument, composed of 'a sphere of 

 thin copper, ten centimetres in diameter, with an aperture on top, in 

 which a thermometer is placed, with its bulb as near as possible in the 

 center of the sphere. The aperture is then closed with gum lac or 

 wax. The sphere is coated on the outside with two coatings of lamp- 

 black, applied with s'ome drying oil, and then definitively attached to 

 the top of an isolated pole. Whenever the sun shines, whatever may 

 be its altitude above the horizon, the sphere always intercepts a lumi- 

 nous fasces of the same dimension, and the ascension of the interior 

 thermometer above the environing temperature, observed in the 

 shade, gives a precise measure of the part luminous radiation bears in 

 accomplishing the phenomena of vegetation. 



ON THE COMPOSITION OF COLORS. 



Hitherto the experiments upon the composition of colors, have been 

 few and unsatisfactory. The fundamental problem is evidently to de- 

 termine the colors which result from the combination of two or more 

 simple colors, which cannot be suitably determined, except by direct 

 experiments made with the colors of the solar spectrum as pure as 

 possible. Newton is the only person w r ho has tried such experiments ; 

 all others who have been occupied with the same subject, P. Castel, 

 Mayer, Lambert, Hay, Forbes, have studied the composition of colors 

 by mixing coloring materials, and it is by experiments of this nature, 

 they have endeavored to show the possibility of reproducing all the 

 colors by means of three simple colors. The evident imperfection of 

 these researches determined M. Helmoltz to study the question anew, 

 making use of the means the modern physicist possesses to obtain the 

 colors of the spectrum entirely pure and homogeneous. 



It is particularly the hypothesis which reduces all colors to three 

 fundamental colors that occnpied M. Helmoltz. This hypothesis has 

 been conceived in different manner by different physicists. 



1st. We may admit simply that by means of three colors we can 

 reproduce all possible colors. 



2d. We may attribute an objective existence to the three funda- 

 mental colors with Mayer and Brewster. 



3d. W r e may say with Young, that the three colors correspond to 

 the three principal impressions of the optic nerve, and their combina- 

 tion produces the other impressions. 



