( 832 ) 
Physics. — “New methods of stereoscopy”’. By Dr. P. H. EyKMan. 
(Communicated by Prof. K. F. WeNCKEBACH). 
In a paper of mine, which was recently published, entitled : “stereo- 
Röntgenography” (Nederl. Tijdschr. voor Geneeskunde, March 18, 
1909), I pointed out that from a mathematical point of view stereos- 
copy by means of the Röntgen rays is much simpler than ordinary 
stereoscopy by means of a photographic camera with lenses. With 
Röntgen rays both the object and its projection are on the same side 
of the centre of projection, whereas with the camera they lie one on 
either side of the centre of projection and moreover the image is 
reversed. With the camera owing to the relative position of the 
conjugate foci the ratio of the distances is fixed. With the Röntgen 
rays this is not the case. With the camera the photographic plate 
must be perpendicular to the principal axis, otherwise the image 
will be hazy in parts. With the Röntgen rays the plate may be at 
any angle, but I have only considered the “normal case’, so as to 
simplify the construction. I pointed out the mistake of applying the 
laws of lens-stereoscopy to Röntgen-stereoscopy, whereas in my 
opinion the better way is to derive ordinary stereoscopy from Réntgen- 
stereoscopy. The so-called ‘‘pinhole-camera” is a transition between 
the two, as in this case the law of conjugate foci does not apply, 
and there is no principal axis. VAN ALBADA understood this, and 
when treating the theory of stereoscopy, placed the object and the 
picture on the same side of the centre of projection, e.g. the observer 
looking out of a window, the window-pane representing the plane 
of projection. For the mathematical reconstruction, the two Röntgen 
plates or their virtual images must be superimposed exactly at the 
same spot, and in the same position with regard to the two anti- 
cathodes. Further since the anticathodes must be replaced by the 
eyes, the distance between the two anticathodes (base of exposure) 
must be equal to the distance of the optical centres of the two eyes 
(visual-base). In practice this distance may be fixed at 65 millimeters. 
The following conditions are requisite for the normal stereoscopic 
exposure : 
1. In the two exposures one plate must be placed exactly in the 
same place as the other, in other words, the plates must be congruent. 
2. The base of exposure must be 65 millimeters. 
3. The nearest point of the object must be at least 25 centi- 
meters from the anticathode, since at a lesser distance the eyes do 
not see stereoscopically. 
4, The base must be parallel to the photographie plate, the middle 
