"The deep study," writes John Harvey Kellogg (1852-1943), "of the sun's rays which has been made by physicists within the last few years, has thrown a great flood of light upon this subject which is of precious value to clinicians. A very important practical fact is the great variability of the intensity of the sunlight and especially of ultra-violet rays, an element of highest interest from a therapeutic stand-point.” (Kellogg, "Light therapeutics", [Battle Creek, MI, 1910], 15). This photograph demonstrates one of heliotherapy's numerous modes of application. Kellogg – of Kellogg's Cornflakes fame – used both natural light outdoors and artificial electric light in his light therapeutics. This early image of an isolated patient undergoing an indoor sunbath depicts a general application of heliotherapy to the entire body through glass panes.