A DEFECT OF PHOTOGRAPHY. 49 



concave, as there shown, were represented by a straight 

 line from c to a. The result would evidently be that the 

 image of the arrow upon it would be shortened by the 

 difference between the arc of a circle and the chord of the 

 arc. This arrangement of the eye, as compared with that 

 of the photographer who is necessarily, for printing, repro- 

 ducing, framing, &c., confined to the use of a flat surface 

 is of great importance when it is remembered that visible 

 space is a concave spherical area of which the spectator's 

 point of observation forms the centre, and that the concave 

 retina is obviously an optical adaptation of the eye to this 

 natural fact for reproducing concavely to the beholder all 

 that part of this concave space which admits of being 

 simultaneously comprehended within the limits of correct 

 vision, and for which no flat surface interposed instead of 

 the retina could form an effective substitute, or one by which 

 the consciousness could be so correctly brougb* in contact 

 with the faculty. The difference between the accuracy of 

 an ordinary mirror (omitting horizontal inversion) and a 

 photograph is that, as a spectator advances to or recedes 

 from the mirror, not only do the rays from the entire area 

 imaged converge at different angles to him and in varying 

 (increasing or diminishing) relative distance from each 

 other, but they also proceed from different parts of the 

 mirror's surface at each change of distance all but the 

 one ray in the line of which the observer is moving. A 

 photograph, from the fixity of its parts, can thus obviously 

 be correct only at one point of distance from it as com- 

 pared with a mirror, even assuming it to be absolutely 

 correct there. Indeed, in contemplating both photographs 

 and pictures, the eye by long habit acquires almost 

 insensibly a new power of adjusting itself, but only to 

 a certain extent, so as to enable it to see these objects 

 in relief, such as the stereoscope gives more fully, which 

 owes its most important merits to an adjusted and fixed 



