I have very recently learned, that, under the present patent laws of the United States, every foreign patentee is required to put his invention, or discovery, into practical use within eighteen months after taking out his papers, or otherwise forfeit his patent. With regard to Mr. Talbot's Calotype patent, this time has nearly, if not quite expired, and my countrymen are now at perfect liberty to appropriate the art if they feel disposed. From the statement of Mr. Wattles, it will be perceived that this can be done without dishonor, as in the first instance Mr. Talbot had no positive right to his patent. Photography; or sun-painting is divided, according to the methods adopted for producing pictures, into DAGUERREOTYPE, CHROMATYPE, CALOTYPE, ENERGIATYPE, CHRYSOTYPE, ANTHOTYPE and CYANOTYPE, AMPHITYPE. CHAP. II. THE THEORY ON LIGHT.--THE PHOTOGRAPHIC PRINCIPLE Some philosophers contend that to the existence of light alone we owe the beautiful effects produced by the Photogenic art, while others give sufficient reasons for doubting the correctness of the assumption. That the results are effected by a principle associated with light and not by the luminous principle itself, is the most probable conclusion. The importance of a knowledge of this fact becomes most essential in practice, as will presently be seen. To this principle Mr. Hunt gives the name of ENERGIA. THE NATURE of Light is not wholly known, but it is generally believed to be matter, as in its motions it obeys the laws regulating matter. So closely is it connected with heat and electricity that there can be little doubt of their all being but different modifications of the same substance. I will not, however, enter into a statement of the various theories of Philosophers on this head, but content myself with that of Sir Isaac Newton; who supposed rays of light to consist of minute particles of matter, which are constantly emanating from luminous bodies and cause vision, as odoriferous particles, proceeding from certain bodies, cause smelling. The effects of light upon other bodies, and how light is effected by them, involve some of the most important principles, which if properly understood by Daguerreotypists would enable them to improve and correct many of the practical operations in their art. These effects we shall exhibit in this and the following chapters. Before we enter on this subject it will be necessary to become familiar with the DEFINITIONS of some of the terms used in the science of optics. Luminous bodies are of two kinds; those which shine by their own light, and those which shine by reflected light. Transparent bodies are such as permit rays of light to pass through them. Translucent bodies permit light to pass faintly, but without representing the figure of objects seen through them. Opaque bodies permit no light to pass through them, but reflect light. A ray is a line of light. A beam is a collection of parallel rays.