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POPULAR SCIENCE MONTHLY. 



or his means may suggest." The author 

 pointed out as a danger in the usual limita- 

 tions of compulsory subjects for examina- 

 tions for certificates and degrees, that in view 

 of them subjects not made compulsory may 

 be neglected, however important to the en- 

 gineer 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 re- 

 cent lecture before the Oxford University 

 Junior Scientific Club, Captain W. de W. 

 Abney gave a very good account of the pres- 

 ent 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 correspond- 

 ing 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 preced- 

 ing, 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 col- 

 oring them alternately red, green, and blue. 

 The negative is taken through this screen, 

 and then developed and viewed through a 

 similaily 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 view- 

 ing, on the contrary, are made as monochro 

 matic as possible. Instead of using trans- 

 parencies 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 gradu- 

 ally 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 de- 

 vised 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 de- 

 posited 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 ap- 

 pearance 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 in- 

 terference 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. 0. 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 intro- 

 duction of wild animals has generally been 

 disastrous, as those of the mongoose in Ja- 

 maica and Australian flying foxes in Cali- 

 fornia. 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 acci- 

 dental importations began with the begin- 

 ning 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- 



