(Blue paper and blue color have been somewhat extensively used by our operators in their operating rooms and skylights, in order to facilitate the operation in the camera. I fancy, however, that this plan cannot be productive of as much good as thought by some, from the fact, that the light falling upon the subject, and then reflected into the camera, is, coming through colorless glass, not affected by such rays as may be reflected from the walls of the operating room; and even if it were so, I conceive that it would be injurious, by destroying the harmony of Shadows which might otherwise occur.) The greatest amount of white light is at C; the yellow contains less of the chemical power than any Other portion of the solar Spectrum. It has been found that the most intense heat is at the _Extreme Red_, _b_. Artificial lights differ in their color; the white light of turning charcoal, which is the principal light from candles, oil and gas, contains three fays--red, yellow and blue. The dazzling light emitted from lime intensely heated, known as the _Drummond light_, gives the color of the prism almost as bright as the solar spectrum. [Sidenote: LIGHT, HEAT, AND CHEMICAL POWER.] If we expose a prepared collodionized plate or sensitive paper to the solar spectrum, it will be observed that the luminous power (the yellow) occupies but a small space compared with the influence of heat and chemical power. R. Hunt, in his _Researches on Light_, has presented the following remarks upon the accompanying illustration:-- [Illustration: Fig. 3.] "If the linear measure, or the diameter of a circle which shall include the luminous rays, is 25, that of the calorific spectrum will be 42·10, and of the chemical spectrum 55·10. Such a series of circles may well be used to represent a beam from the sun, which may be regarded as an atom of _Light_ surrounded with an invisible atmosphere of _Heat_, and another still more extended, which possesses the remarkable property of producing chemical and molecular change." [Sidenote: REFRACTION.]