USAToday has a regular column called "ask the Captain" about aviation matters.
" ... Another very common system which provides visual descent guidance is the Precision Approach Path Indicator or PAPI, pronounced "PAPPY," which is the same idea, but there is just one row of four lights which may be located on either side or sometimes on both sides of the runway. In this instance we want to see the outer two lights white and the inner two lights red. If we see the three outer lights white and the inner one red, we know we're slightly high and make a correction. If we see the three inner lights red and the outer one white, we know we're slightly low and make a correction. Have a look at the following link at airliners.net for a good look at a PAPI on approach into Chicago's Midway Airport (this spectacular photo was taken by Nick Onkow). ... "
Link to the article: http://www.usatoday.com/travel/colum...e/2006-02-20-ask-the-captain_x.htm
Link to Nick's photo: http://www.airliners.net/open.file/0914288/L/