438 OPTICAL PROJECTION 



Omni Focal Lenses. Lantern lenses on the same principle aa 

 the Tele-photo camera lenses of Messrs. Dallmeyer are now to be 

 obtained, and such a lens gives very great adaptability in focal 

 length. 



Messrs. Dallmeyer and Newton have conjointly placed on the 

 market an omni-focal attachment which can be adapted to almost 

 any good double achromatic lantern lens. A 6-inch lens fitted with 

 such an attachment can be used as a 6-inch, 12-inch, or any longer 

 focus required up to about 24-inch. The lantern simply has to be 

 placed at the required distance from the screen, and the lens 

 focussed. If the resulting picture is too large, the draw-tube 

 adapter must be pulled further out, and the lens can then be 

 focussed again in its new position, and a smaller picture will 

 result. 



Unfortunately, up to the present, it is not possible to slightly 

 lengthen the focus of a lens by means of such an addition. The 

 omni-focal attachment, when it is screwed on, about doubles the 

 original focus, i.e. a 6-inch lens becomes a 12-inch lens. Beyond 

 that, almost any length of focus can be obtained. Of course, the 

 attachment can always be taken off, and the original lens used by 

 itself. 



lantern Stereoscope. Desire has often been expressed that 

 the effect of stereoscopic slides could be produced upon the screen, 

 and various attempts have been made without success. Two 

 stereoscopic pictures can, of course, be easily superimposed upon 

 the screen, but for stereoscopic vision some arrangement must be 

 made by which each image can be seen by one eye only, and this 

 on so large a scale introduces considerable difficulty. It had once 

 or twice been suggested that polarised light might solve the diffi- 

 culty ; but nothing, so far as I know, had ever been done in that 

 direction until Mr. John Anderson, of Birmingham, successfully 

 worked out an apparatus, which was publicly exhibited at the 

 Royal Society's summer conversazione in 1893. 



The really new and essential point in this invention is the 

 screen, which is faced with thin silver leaf, varnished to prevent 

 oxidation. This surface, besides its greater power of reflection, 

 possesses the property of reflecting polarised light with its 

 polarised properties, not depolarising it, as most surfaces do. The 

 rest is easily understood. Each slide of a stereoscopic pair is shown 

 from one nozzle of a biunial lantern (or a pair of single ones), and 

 the image is transmitted through a ' pile ' of glass plates, arranged 



