A HALF-CENTURY OF SCIENCE. 201 



no ideas in the mind things which can only be conceived while they 

 are visible ; the intense hollow blue of the upper sky melting through 

 it all, showing here deep, and pure, and lightness ; there, modulated 

 by the filmy, formless body of the transparent vapor, till it is lost im- 

 perceptibly in its crimson and gold." 



But what is the explanation of these gorgeous colors? why is the 

 sky blue ? and why are the sunrise and sunset crimson and gold ? It 

 may be said that the air is blue ; but, if so, how can the clouds assume 

 their varied tints ? Briicke showed that very minute particles sus- 

 pended in water are blue by reflected light. Tyndall has taught us 

 that the blue of the sky is clue to the reflection of the blue rays by 

 the minute particles floating in the atmosphere. Now, if from the 

 white light of the sun the blue rays are thus selected, those which are 

 transmitted will be yellow, orange, and red. Where the distance is 

 short the transmitted light will appear yellowish. But as the sun 

 sinks toward the horizon the atmospheric distance increases, and con- 

 sequently the number of the scattering particles. They weaken in 

 succession the violet, the indigo, the blue, and even disturb the pro- 

 portions of green. The transmitted light under such circumstances 

 must pass from yellow through orange to red, and thus, while we at 

 noon are admiring the deep blue of the sky, the same rays, robbed of 

 their blue, are elsewhere lighting up the evening sky with all the 

 glories of sunset. 



Another remarkable triumph of the last half-century has been the 

 discovery of photography. At the commencement of the century 

 Wedgwood and Davy observed the effect produced by throwing the 

 images of objects on paper or leather prepared with nitrate of silver, 

 but no means were known by which such images could be fixed. This 

 was first effected by Niepce, but his processes were open to objections 

 which prevented them from coming into general use, and it was not till 

 1839 that Daguerre invented the process which was justly named 

 after him. Very soon a further improvement was effected by our 

 countryman Talbot. He not only fixed his " Talbotypes " on paper 

 in itself a great convenience but, by obtaining a negative, rendered 

 it possible to take off any number of positive, or natural, copies from 

 one original picture. This process is the foundation of all the methods 

 now in use ; perhaps the greatest improvements having been the use 

 of glass plates, first proposed by Sir John Herschel ; of collodion, sug- 

 gested by Le Grey, and practically used by Archer ; and, more lately, 

 of gelatine, the foundation of the sensitive film now growing into gen- 

 eral use in the ordinary dry-plate process. Not only have a great 

 variety of other beautiful processes been invented, but the delicacy 

 of the sensitive film has been immensely increased, with the advan- 

 tage, among others, of diminishing greatly the time necessary for 

 obtaining a picture, so that even an express-train going at full speed 

 can now be taken. Indeed, with full sunlight, -g-J-Q of a second is 



