How to Restore Discolored Lenses

      Discoloration of a multiple lens can usually be traced to the cement, which because of age or condition of use may turn yellow, crackle, or become opaque. The average craftsman will not find it difficult to restore such a lens, or to cement elements obtained from salvaged lenses.
      Remove the lens from its mount, running the point of a knife under the rim, if necessary, to spread it. Like most two-lens achromatic combinations, the 16mm. projector unit shown in the photos had a concave and a convex element. They had become separated, so the old cement, which was Canada balsam, was washed off with xylol (xylene), a common balsam solvent. Lacquer thinner, waxless paint remover, and some dry-cleaning fluids work as well. Unseparated lenses may be soaked in solvent or heated to soften the balsam.
      The elements were next cleaned with a commercial lens cleaner. If soap and water are used, avoid strong soaps. Distilled or rain water is preferable to tap water.
      Balsam for cementing lenses usually comes in stick form and must be crumbled and heated to about 300 deg. F. along with the lens elements. A small electric furnace with heat control is ideal (see P.S.M., Nov. 1943, p. HW 554), but a glass or iron frying pan will serve. Heat slowly and uniformly and avoid sudden chilling. If optical balsam is not available, liquid balsam used in making microscope slides can be heated to drive off the solvent. When cool it will be brittle and can be used for cementing lenses.
      Dust the lenses carefully with a soft brush, and try them, if they have not been marked, to match the surfaces. Usually matching surfaces create a slight suction when separated. Lay the lenses on window glass if a furnace or iron skillet is used. Have the concave side of the negative lens up and the matching surface of the positive lens down. You can handle them with photographers' wooden or bamboo print tongs.

      Put balsam on the concave lens--about a 1/4" rounded pile on a 1" diameter lens--and heat gradually until the balsam melts and flattens out into a single puddle. If you can control the temperature, keep it under 350 deg. F.
      Lift the convex lens and place it squarely over the concave one, press down to spread the balsam and slide the upper lens in various directions to get rid of trapped bubbles; then center the lenses so the edges match all around, and press out all excess balsam. Let them cool slowly in the skillet or furnace with the lid in place. Scrape excess balsam from the edges, wash the surfaces with solvent, and polish gently with lens tissue.
      Before replacing the lens unit, clean all other lenses of the system and touch up the black coating of inside surfaces, if necessary, with nonreflecting optical lacquer. Do not clamp lens elements too tightly when replacing them. --MORTON WALLING.