44 ILLUMINATING CALORIFIC AND CHEMICAL POWER OF THE SPECTRUM. 



fracted. That of them the red is least, and the violet most refracted, and, therefore, 

 the white image (z) which the original beam would have given, became elongated so 

 as to form the solar spectrum in which the constituent colours are seen. 



146. Leaving now the optical arrangement by which this phenomenon is produced, 

 let us confine our considerations to the spectrum itself. It is to be remarked that the 

 purity of the constituent coloured spaces becomes greater, as the separation of the rays 

 is more perfect; and, as is shown in jig. Ill, these rays are diverging from one an- 

 other, it is obvious that the farther the screen on which they fall is placed from the 

 prism, the greater will be the apparent dispersion of the rays. If the screen is brought 

 close to the prism, the colours are but little developed, because, not being yet separated 

 from one another, their mixture produces white light; but at a distance of twelve or 

 twenty feet, they have diverged sufficiently, and each one appears for itself. 



147. Inspection proves that, in this spectrum, not only do the various parts differ in 

 respect of the colours they show, but they also differ in their intrinsic brilliancy. A 

 piece of fine-printed paper, held in succession in the different colours, is legible at very 

 different distances. Held in the yellow, it may easily be read at a considerable dis- 

 tance ; but held in the violet, it must be seen close at hand, or the letters cannot be dis- 

 tinguished. The other coloured spaces possess intermediate powers, and, by direct ex- 

 periments made by FRAUNHOFER, it appears that the different portions of the Newtonian 

 spectrum have their order of illumination, as is expressed in the following table, in 

 which it will be perceived that the brilliancy of the yellow rays is taken as unity, and 

 the other rays compared therewith. The letters refer to the figure in the frontispiece. 



TABLE OF THE ILLUMINATING POWER OF THE DIFFERENT REGIONS OF THE NEWTONIAN 



SPECTRUM. 



148. It is not only in illuminating power that these different regions vary ; they vary 

 also in their heating power, as is shown by their action on a thermometer. When a prism 

 made of flint glass is used, and the bulbs of a set of small and delicate thermometers 

 are plunged in the coloured spaces, it is perceived that, commencing with the violet, 

 each thermometer rises higher as it approaches the red region ; and even beyond the 

 red region, where the eye can detect no trace of light, the maximum of heat occurs, 

 thus showing that the heat which exists in the sunbeam is an intrinsically different 

 agent from the light, because, by the action of a prism, it can be refracted, and is found 

 in a space in which no light exists. 



149. It has been known for a long time that the white chloride of silver, and, indeed, 

 all the white salts of silver, when exposed to the sun, turn black (Ap., 443). A piece 

 of paper washed over with any of these white bodies, as the chloride, and held in the 

 spectrum, soon undergoes a change. In the more refrangible region, the rays begin to 

 effect a decomposition, which spreads far beyond the violet extremity ; and when the 

 bromide of silver is used, this darkening action is simultaneously begun from end to end 

 of the spectrum, and chemical action extends beyond both of its extremities. In the 



