POPULAR SCIENCE MONTHLY.
or his means may suggest." The author pointed out as a danger in the usual limitations of compulsory subjects for examinations for certificates and degrees, that in view of them subjects not made compulsory may be neglected, however important to the engineer they may be; and he recommends that a certain very moderate standard in all such subjects should be made compulsory if a certificate of proficiency is to be given in engineering or physical science.
History of Color Photography.—In a recent lecture before the Oxford University Junior Scientific Club, Captain W. de W. Abney gave a very good account of the present state and history of color photography. The first process described was that based on the three-color negatives—three negatives being taken, one through an orange, one through a green, and one through a blue screen. These negatives are developed in the ordinary way, and then viewed through three superposed color screens corresponding with those through which the negatives were taken. This process produces very beautiful results, but is obviously limited in application and is not true color photography. The next process described was that of Dr. Joly, of Dublin, who, basing his work on the same theory of color vision as the preceding, reproduces in color by means of a single negative. The human eye is incapable of separating points or lines which lie very close to one another. Dr. Joly's method utilizes this fact by ruling on a transparent screen lines only one two-hundredth of an inch broad, and very close together, and coloring them alternately red, green, and blue. The negative is taken through this screen, and then developed and viewed through a similarly colored screen, when the picture appears in approximately its natural colors. The viewing differ from the taking screens in both of these processes. The taking screens must not be such as to allow only monochromatic light to pass, but must allow a certain amount of overlapping. The viewing, on the contrary, are made as monochromatic as possible. Instead of using transparencies and colored films, transparent inks may be used to produce pictures by three printings. The oldest process described is that of the production of color by the action of light itself, or the true color photography. Somewhere about 1847 Becquerel found that if, instead of iodizing a plate, he chlorinized it and then exposed it to white light, it gradually assumed a violet tint; and if in this state he exposed it to the spectrum, he was able to obtain the colors of the spectrum on it. Unfortunately, however, these colors were not permanent, and no method has been devised for fixing them. The last method described by Captain Abney was that of Lippmann, who found that if by means of reflection he obtained stationary waves in the film, on development the silver was deposited between the nodes. On reflecting light from such a "noded" plate the proper light alone was reflected, and the photograph, viewed at a particular angle, appeared in its natural colors. If looked at by transmitted light these photographs have merely the appearance of ordinary transparencies. This method is known as the interference method, because the stationary waves which produce the nodes on the plate are caused by the interference of the normal light vibrations.
Distribution of Species by Man.—The Spread of Species by the Agency of Man was the subject of Chairman L. O. Howard's address before the Botanical Section of the American Association. The author showed that while natural spread had been the rule for centuries, the agency of man has become preponderating with the improvement of commercial intercourse between nations. In the intentional introduction of useful plants and flowering plants from foreign countries species sometimes escape from cultivation and become weeds. The intentional introduction of wild animals has generally been disastrous, as those of the mongoose in Jamaica and Australian flying foxes in California. Accidental introductions have been more powerful in extending the range of species and in changing the character of the plants and animals of a given region than intentional introductions. The era of accidental importations began with the beginning of commerce, and has grown with the growth of commerce. The vast extensions of international trade of recent years, every improvement in rapidity of travel and in safety of carriage of goods of all kinds have increased the opportunities of addi-