John,
Another good one John!
Now I'll add yet another!

when trying (pre-digital age) to measure the length of basement 
membrane, I used to make transparent image of section as you 
suggest (actually, I used to project the image up between my legs 
onto a glass screen and trace it).
Then I used a cotton reel attached to a darning needle with the 
thread comming thru' the eye and sticking to the adhesive covered 
image as a I traced along the BM.
When completed, it was easy to strip the thread from the image, 
straighten and measure against a ruler.  
Simple maths then allowed a count of nos. of cells per mm of BM!
Thank goodness for digital imaging, etc.
Russ Allison, 
Dental School
Cardiff
Wales