Memoirs on the Absorption of' Light. 047 



of thickness, but a positive transition from one hue to another. Thus, a 

 solution of sap-green, viewed through small thicknesses, is green, but in 

 great ones is dark red, and numerous other media present this singular 

 transition. 



Mr Herschel proposes the extreme red as a standard ray fo* optical ex- 

 periments, not that exhibited by common red glasses, which is always a 

 mixed colour, but that transmitted with particular facility by ordinary deep 

 blue glass coloured by cobalt. Its place is strictly at the utmost limit of 

 the spectrum, and its refrangibility (when insulated by a combination of 

 such a glass with a red one) definite, as much so as the yellow light above 

 described. Dr Brewster has investigated the effects of heat in changing 

 the tints of media. He has observed different glasses to be differently af- 

 fected by heat, some having their absorbent powers increased, and others 

 diminished — some transiently, and others permanently. Many minerals 

 present similar phenomena. The experiments of Dr Brewster on topaz are 

 well known. The change of colour in many opaque bodies by heat is 

 doubtless referable to this cause. We need only mention minium and the 

 peroxide of mercury, which, at a heat just short of ignition, become al- 

 most black, and recover their bright red hue when cold. 



In both the papers now before us, the insulation of the yellow rays in 

 solar light by coloured glasses, in a state of perfect purity, is regarded as 

 impracticable ; but both these authors have succeeded in so far separating 

 it, as to place the existence of yellow light in the spectrum beyond all 

 manner of doubt, and in showing that the space it occupies is really pretty 

 considerable. Dr Brewster, indeed, regards it as encroaching both on the 

 limits of the red and green, and Mr Herschel attributes to it a breadth 

 not less than one-fourth, the interval between the red and blue. The for- 

 mer draws the conclusion, that the orange and green are really composite 

 colours, which, if verified, would be a fact of the highest importance, in as 

 much as it would prove the prismatic analysis of white light to be imper- 

 fect, and refer the impression of colour on the sensorium to some other 

 cause than that which produces difference of refrangibility. We do not 

 mean to deny this, for, in fact, we think there are other arguments addu- 

 cible in its support. We submit, however, that the celebrated observa- 

 tion of Dr Wollaston, on which the opinion advanced by Dr Brewster is 

 grounded, must have contained some cause of fallacy. He received in 

 his eye the spectrum of a narrow luminous line, and could discern in it no 

 yellow, or so very little as to be attributed by him to a mixture of red and 

 green from the opposite sides of the aperture. 



Nevertheless, if the red and green portions of a long prismatic spectrum 

 be screened, the yellow is rendered very evident ; and in Fraunhofer's ad- 

 mirable experiments, (which we had the pleasure of witnessing in perfec- 

 tion at Munich through his kindness,) where, from the exquisite limpidi- 

 ty of the prisms used, and the delicate adjustment of his whole apparatus, 

 the absolute homogeneity of every part of the spectrum is fully assured, the 

 orange, the yellow, and the green, are all seen shading into each other by 

 insensible gradations ; the yellow being remarkably conspicuous, and of a 



