way down the tunnel. you will find a yellow and blue crystal in the prism on the floor.
49. go through the to tunnels to the right and left of the center platform and get the triangle and square prism use the blue and yellow prism to get them.
50. go back to were you got the blue and yellow prism and place the triangle and square in the triangle and square boxes. then uses the touch on the triangle and blue prism.
51. go through the tunnel where you climbed on the bridge from and get the octagon object from the prism on the ground and bring it back to the basin, 