248 Laws of Metallic Reflexion, &c. 



oimen which has furnished the values of M and \. There is 

 one metal, however, with respect to which there can be no 

 doubt that the experiments of different observers are strictly 

 comparable, when it is pure, and at ordinary temperatures I 

 mean mercury. For this metal Sir David Brewster states the 

 angle of maximum polarization to be 78 27', and the maximum 

 value of /3, when a = 45, to be 35 ; from which I find M = 4-616, 

 X = 68 13', and, at the perpendicular incidence, I = '734. 

 Now Bouguer observed the quantity of light reflected by mer- 

 cury, but not at a perpendicular incidence. His measures were 

 taken at the incidences of 69 and 78|- , for the first of which 

 he gives, by two different observations, '637 and '666 ; for the 

 second, by two observations, -754 and '703, as the intensity of 

 reflexion.* If we make the computation from the formula, 

 with the above values of M and ^, we find the quantities of 

 light reflected at these two incidences to be, as nearly as pos- 

 sible, equal to each other, and to seven-tenths of the incident 

 light, the intensity of reflexion being a minimum at an inter- 

 mediate incidence ; and if we suppose these quantities to be 

 really equal at the incidences observed by Bouguer, we must 

 take the mean of all his numbers, which is '69, as the most pro- 

 bable result of observation. This result differs but little from 

 one of the two numbers given by him at each incidence, and 

 scarcely at all from the result of calculation. 



The angle at which the intensity of reflexion is a minimum, 

 when common light is incident, may be found from the formula 



which gives the values of p., and thence that of i. This inci- 

 dence for mercury is, by calculation, 75 15', and the minimum 

 value of / is '693, which is less than its value at a perpendicular 

 incidence by about one-eighteenth of the latter. According 

 to the formulae, the reflexion is always total at an incidence 

 of 90. 



* See his Traite ffOptique sur la Gradation de la Lumihe : Paris, 1760 ; pp. 124, 

 126. 



