Pièce 20 - James David Forbes to William Whewell

4 pp

Cambridge - JDF gives his explanation for the transition from light to heat at the red end of the spectrum: 'Assume it to be demonstrated that both Light and Heat consist of transverse vibrations of one and the same medicine. Assume that vibrations beyond a certain length cease to affect the eye, but produce the sensation of heat, which shorter ones are unable to do'. The greatest problem is to account for the presence of heat accompanying the light: 'That there is a general connexion between the length and the velocity (and consequently the refrangibility) of a wave Mr Cauchy [A.L. Cauchy] is said to have made out'. JDF has not looked at his results sufficiently to know even if the supposition is correct: 'for instance if it be (as I rather think Mr Cauchy makes it) dependent on Trigonometrical quantities we may have an indefinite number of specific lengths of waves which shall answer the condition of a specific degree of refrangibility, and amongst these only one may be contained within the limits producing the sensation of Light whilst a certain number may belong to the department of heat, and those above a certain length may have no existence in nature, as may be imperceptible to all our senses'. Another important consequence of this theory is the mixture of heat of different qualities: 'When examined by absorbents various kinds of heat are to be found in every ray of the spectrum, in short there is (probably) not a homogenous ray of heat in it'. JDF suggests: 'If we limit the law of dispersion to giving only two numerical values of the length of waves (though our hypothesis requires more) we may have the intensity of heat expressed as below [a graph representing the heat of a spectrum] by the sum of the ordinates of a curve extending the length of the spectrum and folding back on itself (supposing as we must do that the heat increases to a certain point with the length of wave, since a short wave is only light)'.