It's like trees on a golf course, the rotor and stator discs in the turbine are 75% air and when they line up you can see straight through.
Thanks, Hobo. I suppose I'd assumed that turbine disks were like fan disks, less than 50% air, and that therefore one would never be able to see past both rotor and stator.
Shorly after starting up the 707's flames became invisible. Would this be because the flames themselves had reduced?