| 摘要 |
<p>In the prism of the invention, the Ra value is less than the value corresponding to &num;170 for a surface that serves as a light incidence surface after forming, and a glass parent material having a predetermined size ratio is gripped by a feed means and fed into a furnace, where it is heated to a temperature such that the lowest viscosity is not less than 104 Pa·s but less than 106 Pa·s, the lower portion being stretch-formed by a stretch means and cut into long bodies having a desired size and an incident light surface whose surface Ra value is not more than 1/4 of the incident light wavelength, and then such long body being cut into pieces of desired length, thus providing prisms.</p> |
