WHAT ASPECT OF THE PROBLEM WAS MOST DIFFICULT? Secondary color related correction, spherochromatism, etc.; transition from fictitious glass representations to real glasses was more difficult than usual. WHAT INSIGHTS (IF ANY) DID YOU GAIN FROM WORKING ON THE PROBLEM? I was surprised to see how good a design could be achieved without air spaces (the use of high index materials helped a lot). On the other hand, we are fortunate that normally this is not a requirement. ADDITIONAL COMMENTS: Interesting problem!