416 On the Stereoscopic Representation of Print. 



generally. In the imitation of paper money the criterion of 

 comparison has always been the difference of form. The pro- 

 cess above suggested affords a much surer test. If, for instance, 

 a bank note and its copy be laid side by side in the stereoscope, 

 a difference in the position of the words, invisible to the naked 

 eye, becomes immediately obvious, as above mentioned, by an 

 apparent projection out of the plane of the paper. By this 

 means, also, a simple and effectual method is obtained of recog- 

 nizing a copy of a print or drawing as such. The publication 

 of the process has, it is true, the disadvantage of putting into 

 the hands of those who design to forge such copies, the means 

 of testing, by the stereoscope, how closely their work resembles 

 the original ; but the difficulty, even with this assistance, of pre- 

 serving a resemblance bordering on identity is so great, that it 

 would rather have the effect of a warning, since the hope of suc- 

 ceeding in making an exact copy is rendered so remote. 



The effect of dampness on paper can be ascertained by moist- 

 ening one of two proofs of the same sentence placed together in 

 the stereoscope. Should there, on the contrary, be a difference 

 between two proofs of the same sentence due to unequal dryness, 

 they may be made to resemble by wetting both. The influence 

 of temperature — on copper-plates, for instance — may be detected 

 in the same manner, the stereoscope thus playing the part of 

 hygrometer and thermometer. 



For the purpose above explained, either the stereoscope should 

 have no bottom, so that it may be placed immediately on the 

 papers to be compared, or, in the place of the four perpendicular 

 dark edges of the table at the bottom, slits should be cut, through 

 which long strips of the proofs to be compared may be drawn, 

 and examined together piece by piece. 



If Slides I. and II. be placed at the same time in an ordinary 

 Wheatstone^s mirror-stereoscope, one on the right side, the other 

 on the left, the alternate projection of the repeated lines is obvious 

 and is of such a nature that the lines which in the one image 

 are those that project, are those which in the other appear de- 

 pressed : the letters, however, here appear placed like type. Those 

 who find a difficulty in reading letters so placed, may make use 

 of a Wheatstone's mirror-stereoscope, together with my prism- 

 stei'coscope with two prisms (Pogg. Ann. vol. Ixxxiii. p. 186, No. 

 4), which Moigno has called the Stereoscope a reflexion to tale, 

 T\Tieatstone the Pseudoscope. In order to detect the dissimilar 

 parts of large plates, the Wheatstone mirror-stereoscope must be 

 so adapted as to admit of plates of whatever size may be desired. 

 This is easily effected by fastening the reflectors, not between 

 two boards, but to a single one, and by leaving only the under 

 groove for the support of the object, the upper groove being 



