L I G 



their intensities at, any assumed unit of distance, and let n = A B ; them 

 it may bo shewn that the required surface, is a sphere of which the ra- 



dius ~ / M , and whose centre has for an abscissa - . 

 n m v m ' m 



Cor. If m n the radius is infinite, as also the abscissa from the cen- 

 tre ; in this case the surface is a plane perpendicular to the middle of the 

 line A B. 



LigJity velocity of. 



2. Light takes up about 16 minutes in passing- over a space the dia- 

 meter of the earth's orbit, which is nearly 190 millions of miles; .'. it 

 travels at the rate of almost -200,000 miles per secnod. 



iminution of y under various circumstances. 



3. If the spaces through which light passes through a uniformly dense 

 diaphonous medium increase in arithmetical progression, the quantity 

 will decrease in geometrical progression. 



Let the space be divided into equal portions or laminae, and suppose 

 th part of the whole light to be lost or absorbed in its passage thro' the 1st 



lamina ; then = quantity of light entering the 2d lamina ; 5 

 rr do. entering the 3d ; ^ rr do. entering the 4th, &c. 



TABLE from Buttguer, shewing the intensity of the sun's light at differ. 

 ent altitudes y and the thickness of air it has to penetrate at each angle. 



1G4 



