rBICAI npTICS. 



rs, and making an angle with tlieir line 



of it.' i will l.c '2 MII~* (sin.0 sina); a l'in^ 



the angle between the mirrors. 



1600. Two prisms of equal refracting angles arc placed with 

 one face of each in contact, and their other faces parallel, and a 

 ray passes through the combination in a principal plane : prove 



its deviation will be from the edge of the denser prism. 



1601. If r t s be the radii of the bounding surfaces of a lens, 

 fa thickness be fl + W-rJ, all the rays incident on the 



lens from a certain point will pass through without deviation and 

 ut aberration. 



1 002. What will be the centre of a lens whose bounding sur- 

 faces are confocal paraboloids having a common axis ? Prove that 

 the distance between the foca of the lens is 



4a, 46 being the latent recta. 



If tin- i>:i(h of a ray through a medium of variable 

 density be an arc of a tin- piano of xy, the refractive 



index at a point (x, y) will be 



x a J \y 6> 

 / being an ar met ion, and (a, b) the centre of i 



iy of Ifct. <.f 



i divide* the medium syrm 



ve that the path is such that, if described by a 

 velocity always proportional to p. the index of i 

 the accelerations c>t 1 to two rectangular axes x 



i will be proportional to -^ , -~- respectively. 



