In order to make the water particularly effective, you could also use a burning glass of at least 1.5 schuh (45 cm) diameter with a focal distance of, at the most, 3 feet and with it direct the concentrated sunrays, as they appear in the focal point onto the water in such a way that the focal point, depending on the height of the vessel, falls 1, 2 or up to 3 zoll (1.5 to 4.5 cm) below the surface of the water, that is, more in the center of the water mass. However, the focal point must not be more than half a minute in the water as thereby the certain invigorating spirits of the water would be captivated too much by the light spirits from the sun, and with weaker natures such a water would produce a too violent effect which would be detrimental rather than beneficial to the subsequent main cure. – The Healing Power of Sunlight, Chapter 2, Paragraph 8