This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
919. Keep the surface cleaned, using a soft cloth, such as an old washed-out linen handkerchief. Dust the surface lightly at first, to remove the larger particles; then polish carefully until perfectly clean, dampening the lens by breathing upon it, if necessary. Never use any sort of polishing material on the lens. Never allow strong sunlight to fall upon the lens for any length of time. Protect the lenses against sudden changes of temperature as much as possible, and where moisture condenses upon them from this cause it should be immediately removed.
920. Should it be necessary to remove the lenses from their mountings to clean them, which should be done frequently, remove one at a time, clean and replace it exactly as found in the cells; otherwise the correction of the lens may be spoiled.
921. In case the lens becomes damaged accidentally, or the surface scratched or coated by the actions of the chemicals in the air of the studio, it should be returned to the makers for repairs.