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3.625" dia.(exactly) Optical Crown,BK-7 Plano/plano x 5/8th" thick..Error is a large, edge chip.. Condition is Used. Shipped with USPS Priority Mail.Tell you what~~This Crown piece could be great to make your own Medium-Large+(3.625" Standard,size) Achromat Refractor lens. I will tell you? It needs a second lens element(Too make a Achromatic lens doublet lens type)..    What you need (you'll 'want' to acquire a special 'Flint' glass type~~Called SF-5(Or SF-05) Basically the same but a highly accurate glass type piece..Not very expensive..This element 'Corrects' the first 'Crown' Elements'(Bk-7) Color Abberations(Inherent in all forms of Crown glass)..A second element is needed needed also, to correct the Focus Abberation(Small tho, also Inherent in single lens properties)...
There are lots of Wholesale Glass Warehouses who supply this needed Flint glass..Just ask for SF-5 (Sf-05) and ask that it is Fine annealed(Keeps ripples out of the glass),which can cause havoc in the lens' performance.Just 'Fine-annealed' is justice.. 
Newport Glass Company or, United Lens Co., or O'hara glass-- or a number of good sources> through the Country..I will help you with your choice of Companies & Glass..k?Just ask Me..
Later,
Philip

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