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THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



In the first years which followed Daguerre's discovery, the ablest 

 operators, in spite of the most careful manipulation, frequently failed, 

 and seldom obtained proofs possessing the clearness of the image as 

 seen on the unpolished glass. At first, this was supposed to result 

 from the difficulty in making the surface of the plate coincide accu- 

 rately with the unpolished surface of the glass. A photographer, 

 M fc Claudet, sought to remedy this inconvenience, and he succeeded. 

 But the result was contrary to his expectation. The proofs obtained 

 were stilt confused and ill-defined. After fresh researches, he dis- 

 covered the cause of non-success : it was, that the focus of the visible 



FIG. 220. Simple object-glass. 



FICJ. 221. -Complex object-glass with adjusting-lens. 



rays of light does not coincide with that of the chemical rays the 

 photogenic focus. And this difference depends on the nature of the 

 glass employed, the distance of objects, and the intensity of the light. 

 The problem has since been practically solved by opticians, who 

 construct object-glasses in which the photogenic and visual focuses 

 coincide. When the object-glass of an apparatus has not this pro- 

 perty, it is important that the photographer should study it with 

 care, and, by multiplied attempts, succeed in finding the exact 

 position of the frame in which the inmge on the rough glass will be 

 found to coincide with the chemical focus, so as to produce the most 

 accurate image possible on the sensitized glass. 



We have said nearly all that is necessary, in a scientific point of 

 view, on this interesting application of physics and chemistry to the 



