\62 PROPERTIES OF LIGHT. 



traversed by a solar ray, without any of its particles being 

 reflected; which furnish«3 means of measuring with accu- 

 racy the quantity of light, that these substances absorb; a 

 problem, which the partial reflection had rendered impossi- 

 ble to be solved. 

 and from The light that has undergone tliis modification comports 



bodied !'° '^ ^ itself in a similar manner with opake polished bodies. 

 Under determinate angles it ceases to be reflected, and is 

 totally absorbed, while within and beyond these angles it is 

 in part reflected from the surface of these bodies. 

 Direct ray on When a solar ray is made to fall on a polished glas«5, that 

 an unsilvered jg „(,t silvered, this ray is in part reflected at the first and 

 glais, , ,. 1 • • • . • , , 1 



second surtace, and us intensity increases with the angle 



of incidence, reckoning from the perpendicular: in other 

 words, it is so much tlie greater, in proportion as the ray i» 

 more inclined to the reflecting surface. 

 Light previ- But if the direct light be subject to this law of intensity, 



ou^ly reflected t^.^t which has been already reflected follows a very dif- 

 ferent Uw. ferent law, when it is reflected anew by a second glass. In 

 certain directions, Instead of increasing in intensity with the 

 angle of incidence, on the contrary it diminishes ; and, after 

 having attained a certain minimum, begins to increase ac- 

 cording to the same law as the direct light. These minima 

 . are relative either to the inclination of the ray to the re- 

 flecting surfaces, or to the angles which these surfaces form 

 with each other, so that th« li^^ht reflected by the second 

 glass is a function of these three angles. This function ha» 

 an absolute minimum ; that is to say, a point at which the 

 intensity of the light reflected by the second glass is alto- 

 gether null. Calculation has led me directly to the cir- 

 cumstances, that produce this minimum; and I have veri- 

 fied it by a very simple experiment, which I shall proceed 

 to describe. 

 , If we take two glasses inclined to each other at an angle 



which all the of 70' 22' : if we then conceive between these two glasses a 

 JjS^'JJ^e^^^J^'!^ line making with each an angle of 35° 25', every ray re- 

 absorbed by a fleeted by one of the glasses parallel to this line will not be 

 *erorut. reflected anew by the second; it will penetrate it, without 



any of its particles experiencing the action of the repulsive 

 forces, that produce the partial reflection. Within or be- 



youd 



