Set of nine Gratings and Slit slides in 50 x 50mm mounts. Six of the nine slides are multiple slit slides which have 0.08mm lines with 0.17mm space.The other three slides are coarse gratings with the following features:Coarse Grating 1: 150 slits 4 lines / mm with 0.08mm line and 0.17mm space.Coarse Grating 2: 150 slits 4 lines / mm with 0.04mm line and 0.21mm space.Coarse Grating 3: 300 slits 8 line / mm with 0.03mm line and 0.10mm space.(Please note: all slit dimensions are approximate only as this can vary slightly depending on the materials which are currently available at the time of production)