134 Mr. J. M. Davidson on X-Bays and Localisation. [April 25, 



state that there is also exhibited there an instrument for obtaining 

 the stereoscopic image directly on the fluorescent screen. 



On two occasions at the Soiree of the Royal Society I have 

 exhibited apparatus for giving the stereoscopic picture on the fluo- 

 rescent screen, but, while interesting, they were too cumbersome for 

 practical use. The problem to be solved mechanically was quite 

 definite. Two tubes had to be placed opposite the observer's two 

 eyes, the distance between the anodes preferably should be the same 

 distance apart, namely about 6 centimetres, as the distance between 

 the observer's two eyes. A fluorescent screen interposed midway had 

 to be illuminated by one tube, and the shadow of whatever object was 

 cast upon the screen had only to be observed by one eye, the other 

 being eclipsed, then the next tube had to be illuminated and then the 

 other eye only had to see the shadow produced by it. If this alter- 

 nating arrangement could be repeated quicker than ten times a second 

 the impression of the two shadows becomes continuous in each eye, 

 the consequence being that the combined mental image stands out in 

 striking stereoscopic relief. This diagram shows the principal fea- 

 tures of the instrument which I am exhibiting in the Library. I 

 have to thank my friend Dr. Muirhead for the help he has given me 

 in the construction and designing of this instrument. A previous 

 one was made for me by Mr. Crawley, representing Messrs. Muirhead, 

 but which depended for its action upon two synchronous motors, but 

 the latter instrument with the oscillating eye-piece is likely to super- 

 sede the former. 



The practical value of getting a clear and brilliant stereoscopic 

 image upon the screen is immense, for not only are the parts seen in 

 their correct position, but when the apparatus is properly arranged, it 

 is possible for the observer, with a metal probe, needle or forceps, to 

 touch any object he desires which he sees in the stereoscopic image. 



[J. M. D.] 



