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LXVIII. Stereoscopic rejn'esentatiun of Print as it appears when 

 viewed with both eyes through Double-refracting Spar. By 

 Prof. H. W. Dove*. 



IN the Report of the Academy, 1858, p. 315, andPogg. Ann. 

 vol. civ. p. 329, I have stated that if a plane drawing be 

 regarded with both eyes through a crystal of calc-spar, one image 

 appears elevated considerably above the other, while if it be 

 regarded with one eye, the two images appear to lie in the same 

 plane. As the reason for the elevation in the former case is to 

 be sought in the different refraction of the ordinary and extra- 

 ordinaiy rays, I came to the conclusion that the phsenomenon 

 in the calc-spar would be reproduced stereoscopically if the 

 double refraction were represented by a double impression, the 

 different refraction of the two rays being represented by a shifting 

 of the repeated line towards the first line. The six top lines of 

 the accompanying illustration, when regarded in the stereoscope, 

 display this phsenomenon in a striking manner ; the last line 

 relates to the following paper. 



If, in a stereoscope, the drawing designed for the right eye be 

 substituted for that for the left, and vice versa, the convex relief 

 becomes concave. It is obvious that if it be desired to render 

 this change visible, if, for instance, in the case of a truncated 

 pyramid, we wish to observe the passage of the sectional surface 

 through the base, the change must be so managed that, during 

 the period in which it takes place, both projections may remain 

 in the field of view. This can be effected most simply, if a drawing 

 be regarded with one eye naked and with the other through 

 a reflecting prism, and at the same time be turned through 

 an angle of 180°. I have described this in the Report, 1851, 

 p. 249, and Pogg. Ann. vol. Ixxxiii. p. 185. If it be desired to 

 apply this principle to an ordinary lens- or prism-stereoscope, 

 it is only necessary to fasten the two drawings to equal rotating 

 circles, and to set them turning by means of cross strings. As 

 the phsenomenon announced by me has since been exhibited by 

 many physicists, it is possible that this contrivance also has 

 been thought of. The last modification, as far as I know, 

 has been published by Henry Halskef, who has shown that by 

 means of complicated mechanism for moving both images in a 

 direction parallel to the line uniting the eyes, the motion of the 

 section perpendicularly down upon the plane of the drawing may 

 be made visible. It is, however, easy to effect this by the motion 

 of a single image. To give an idea of this, I have had the first 



* From Poggentlorff's Annalen, No. 4, 1859 ; commuuicatcil by tlir 

 Author. 



t Pogg- Ann. vol. c. p. 657. 



