A Manual Of Photography

THE AGENT PRODUCING CHEMICAL CHANGE. 121 Notwithstanding the amount of light impinging along the line covered by the spectrum, no change whatever takes place on the paper. Preserving the prism a, Pig. 15, the yellow glass 5, and the paper c, in the same positions, place a mirror at d, so that the sunbeam is strongly reflected on the paper : it will he blackened over every portion except that upon which the spectral image falls: along this line the paper will still be preserved white and unchanged. Thus we obtain conclusive proof that it is not LIGHT, luminous power, which produces the chemical change. That it is not HEAT is shown in the same manner by the protect- ing influence exerted by the maximum calorific rays; and there- fore we are driven to the hypothesis of the existence of a new agency—a new imponderable element—or a novel form of force which is broadly distinguished from these principles or forces in its effects. To mark this the term Actinism has been proposed, and it is now very generally adopted. The word signifies nothing more than ray power, and therefore, as involving no theory, it is free from many of the objections which would apply to any other term adopted from preconceived ideas. 16. (Space of extra spec- ( tral blue. 6 Lavender. B Violet. Indigo Blue. Green. Yellow. Orange. Red. A a Extreme Red Photouranhv appears to be a misnomer, since the pictures so called are not drawn by fight. It is, however, too firmly rooted