On the Stereoscopic Representation of Print. 415 



lines on Slide II. so printed, that now the odd Hues, instead of 

 the even ones, are shifted in. The lines that before appeared in 

 the stereoscope depressed, now appear raised. It is clear that 

 if the repeated Hnes were made moveable on a slide, the motion 

 perpendicularly down upon the plane of the picture would be 

 immediately obvious. 



As the double-refracting power of calc-spar decreases with its 

 increasing temperature, while its rhomboidal form approximates 

 continually to the cube, if this change could be carried very far, 

 the negatively double-refracting calc-spar, after passing through 

 the state of single refraction, would become at last positively 

 double refracting, while the extraordinary ray, which at the 

 common temperature exhibits a less refraction than the ordinary 

 ray, would then exhibit a greater refraction. The experiment 

 above described explains what would appear with relation to the 

 visible motion of an image seen in a crystal of calc-spar subjected 

 to a continually increasing temperature. 



k 



LXIX. On the Ajjplication of the Stereoscope to distinguish Prints 

 from Reprints, or generally Originals from Copies. Bii Prof. 

 H. W. Dove. 



THE considerable degree to which the lines, in the ac- 

 companying illustration appear in the stereoscope to be 

 elevated one above another on a comparatively trifling shifting 

 of the lines themselves towards each other in a horizontal direc- 

 tion, shows that we have hereby a means of rendering strikin"-ly 

 visible the difference between prints not absolutely identical. 

 It is obvious that if the interspaces of the individual words are 

 not absolutely equal in the two printed impressions, those which 

 to the naked eye appear in one plane, will (in the stereoscope) 

 rise like steps one over another. The lowest lines of the two slides 

 have been composed of the same type, without the compositor 

 being told that a difference was required, and yet, although the 

 difference of the distance between the second and third words is 

 imperceptible to the naked eye, in the stereoscope the three 

 words rise like steps one over the other — the first word being 

 the lowest, the second higher, the third highest. While, there- 

 fore, a re-imprcssion of the sentence from the same form appears 

 all in one plane, a reprint of the same sentence, even though from 

 the same office, and though the greatest care has been taken to 

 preserve the resemblance, will present a perceptible difference. 

 Whether, also, in a new edition only the title has been changed, 

 can be easily determined in the same manner. 



What has been said of printing applies of course to copies 



