THE CAMERA OBSCURA. 



photography also renders it necessary that the lenses should be 

 achromatic or nearly so. These conditions, as well as the chemical 

 processes by which the daguerreotype-plate or photographic-paper 



Fig. 5. 



is prepared before receiving the picture, and treated after its 

 reception, will be more fully explained in another Tract of this 

 series, which will be expressly devoted to Photography. 



208 



