| http://www.w3.org/ns/prov#value | - Further, there are a method of forming a diffraction grating by directly cutting glass, and a method of forming a diffraction grating of the finely-stepped shape by wet-etching or dry-etching SiO2 or the like. [0066] As is apparent also from FIGS. 4 and 5, since the non-effective surface 3 of the diffraction grating 2 is a part of the conical surface, if the sign of the refractive power P of the l
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