Normally, you must provide appropriate spacing between aircraft approaching an airport even on visual approaches. However, if an aircraft reports another aircraft in sight which he will follow, he may be instructed to follow that aircraft and cleared for a visual approach (remember that if a pilot reports the preceding aircraft in sight, he does not have to report the airport in sight for a visual approach). In this case, the pilot is expected to take appropriate steps to sequence himself with that aircraft. You may use this to set up a “string” of aircraft, each knowing who he’s supposed to follow, all cleared for a visual approach. Essentially, each pilot is maintaining visual separation from the one he is following. If an aircraft will follow a heavy or 757, then you must inform them of the type aircraft they are following; however, you are not required to provide any wake turbulence separation; that is up to the pilot.

{Reference: 7110.65 7-4-3-c-2}