What was found
(CAN) AC WAS IN LEVEL FLIGHT AT FL190. CABIN ALTITUDE WAS 5500 FT, NEAR MAX DIFFERENTIAL OF ABOUT 5.5 PSID. SLIGHT THUD WAS HEARD FROM PASSENGER COMPARTMENT. NO DECREASE IN PRESSURIZATION OR ANY OTHER WARNINGS. FLIGHT CONTINUED WITH NO PROBLEMS. PASSENGER SEATED IN EMERGENCY EXIT SEAT REPORTED THAT HIS WINDOW HAD CRACKED IN MID FLIGHT. ON EXAMINATION BY MAINT, OUTER PANE OF TWO PANE CABIN WINDOW HAD CRACKED FROM TOP TO BOTTOM ON A DIAGONAL. CRACK APPEARS TO HAVE STARTED AT A CRAZE( 1.7 IN) IN CENTER OF OUTER PANE. PORTION OF WINDOW WITH CRAZE WAS THINNER THAN THE REST. TWO PANE CONCEPT WORKED PERFECTLY, WITH INNER PANE TAKING PRESSURIZATION LOAD W/O SIGNIFICANT PRESSURE LOSS.