If you cannot find a legitimate copy of Kerr or want to supplement it, consider these :
This is the "cookbook" section that makes the PDF so valuable for lab work. Optical Mineralogy Paul F Kerr.pdf
Paul F. Kerr’s Optical Mineralogy is widely regarded as a classic textbook. It serves as a bridge between theoretical crystallography and practical petrography. Unlike some modern texts that rely heavily on color photographs, Kerr’s work is prized for its rigorous explanation of the physics of light interaction and its comprehensive descriptive tables. If you cannot find a legitimate copy of