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All these instruments, together with many others, will be 

 described in the present division of the work, and their 

 analogies with Nature shown. 



We will now take the subject of the Camera Obscura, an 

 instrument with which the photographic apparatus of the 

 present day has made most of us familiar. As its action 

 depends chiefly upon the glass, or lens, through which the 

 rays of light pass into the instrument, we will first explain 

 that. 



A " lens " is a glass formed in such a manner that the rays 

 of light which pass through it either converge to a focus, or are 

 dispersed, by means of the law of refraction. Every one who 

 has been photographed — and who has not ? — will remember 

 that when the sitter has taken his position, the photographer 

 brings to bear upon him a circular glass fixed into a short 

 tube, and then looks through the instrument as if he were 

 taking aim with some species of firearm. It is no matter of 

 wonder that when savages see the photographic camera for the 

 first time they are horribly frightened, for there is really 

 something weird-like in the appearance of the lens thus pre- 

 sented. 



Now, this lens is of the shape called " double convex," both 

 sides being equally rounded, so that a section of it would be 

 shaped very much like a parenthesis (). The effect of this 

 form of lens is to bring the rays of light to a point at a given 

 distance from the centre. This point is called the " focus," 

 and is well known by means of the common burning-glass, 

 which will set fire to objects placed in its focus, while itself 

 remains quite cool. 



I have seen lead pour down like water when placed in the 

 focus of a large burning-glass, and even the harder metals will 

 yield to the power of the sun's rays when thus concentrated. 



There is nothing which gives a more vivid idea of the 

 amount of heat thrown on the earth by the rays of the sun 

 than the effects of a moderately large burning-glass — say one 

 of six inches in diameter. Taking a circle of this size as the 

 surface of the earth, it does not seem as if any very great 

 amount of heat can be received, but when we catch the rays of 

 that circle in our glass, and bring them together upon the 



