JULIUS RHEINBERG ON STEREOSCOPIC EFFECT AND A 



introducing a certain amount of compensation for any distorted 

 depth dimensional effects. 



With regard to the best place to have the stops, there are 

 i ious possible places. They are, 1, immediately in front of the 

 objective ; 2, between the lens and the Wenham or Bidclell 

 prism ; or 3, above the eye-pieces in the plane of the Ramsden 

 circle. Theoretically the last-named is the best place, and, 

 :perimenting with the Abbe stereoscopic eye-piece, I have 

 obtained good results with the stops in this position; but owing 

 to the minute size of the Ramsden circle, and the consequent 

 extra difficulty of adjustments, that position cannot be recom- 

 mended for other forms of binoculars. Immediately in front of 

 the lens is a very convenient spot, but from certain experiments 

 made does not appear to act quite so well as when the stops are 

 behind the lens, and I have obtained the best results by having 

 them immediately below the prism. I suggest that opticians 

 will find it the most convenient plan to provide a sliding carrier, 

 allowing the stops to come as near to the undersurface of the 

 Wenham or Riddell prism as practicable. 



Mr. Taverner has pointed out to me a concomitant advantage 

 of a practical nature, which follows if binoculars are provided 

 with the stop-carrier in the position named. At present most 

 binoculars are provided with a fixed stop just below the prism, 

 the diameter of which is too small to allow the full aperture of 

 low-power objectives with large back lenses — i.e. of comparatively 

 luge N.A. — to be utilised. This is an inconvenience, particularly 

 when, as is frequently desirable, the binocular is used as a mono- 

 cular instrument. The purpose of the fixed stop seems to be partly 

 to cut off light reflections, but chiefly, probably, because it would 

 be found that penetrating power would not be great enough if it 

 were larger. If a stop-carrier as suggested is employed there is 

 no need for the fixed stop, and the instrument may be used as a 

 monocular, utilising the full N.A. of any objective, such as a |-im 

 apochromatic. 



3 a well-known practical fact that stereoscopic binoculars 



only act well with low and medium powers, and that they fail 



with high powers— at all events, comparatively speaking. It 



been found by Stephenson and Abbe that the best stereo- 



results are obtained by the use of a stop with two circular 



tures m the condenser, so that two separate cones of light. 



