BCIOPTICON MANUAL. 



21 



P. 



action of the crystalline lens and the vitreous and 

 aqueous humors. As the retina, R R R (Fig. 11), is con- 

 cave, the centre of concavity being the optical centre, 

 there is no need of adjusting the focus to a flat field. 



The eye is readily, for the 

 most part unconsciously, ad- 

 justed, so that an object upon 

 which we fix our attention is 

 at once in the centre of the 

 field of view, and is focused 

 according to its distance. 



These five troublesome properties enumerated in this 

 chapter, are thus, in the eye, harmoniously reconciled. 

 In art we lack the peculiar crystalline lens, and the con- 

 cavity of field. Making amends for this lack interferes 

 with other corrections. Efforts of various makers to 

 effect the best compromise for particular kinds of work 

 has given rise to lenses, in variety too numerous here to 

 particularize. 



THE LANDSCAPE LENS. 



This simple achromatic lens (Fig. 12) is the oldest 

 photographic lens in existence. It is composed of the 



. 12. 



concave lens of flint-glass /, and the convex lens of 

 crown-glass c. 



