E. M. NELSON ON BINOCULARS. 51 



designed as a non-stereoscopic binocular for use with liigli 

 powers ; if Dr. Mercer's method of racking in or out the tubes 

 is used " ortho- " or " pseudo- " stereoscopic vision will be 

 obtained. The conditions are precisely similar to those of 

 Abbe's ; repetition is, therefore, unnecessary. 



Stephenson.* We may best consider this as a Holmes' 

 " sawn in two " binocular of the pseudostereoscopic type, used 

 in conjunction with a pair of Wheatstone's pseudoscopic 

 spectacles. (Wheatstone's pseudoscopic spectacles consist of 

 two transposing prisms.) Stephenson's prisms are similar to 

 Wheatstone's, and neutralize the transposition of the images 

 by the eye-pieces, thereby restoring "the ordinary view," with 

 the exception of the inversion, which is left, and which does 

 not influence the stereoscopism at all. In most binoculars of 

 this type there is another prism to correct the inversion ; the 

 resultant image, therefore, is erect, and not transposed, but 

 this binocular is orthostereoscopic whether the image is erect 

 or inverted. This ingenious binocular was first invented by 

 Riddellf in 1851, was shelved, and independently reinvented 

 by Stephenson in 1870. In passing, let me say that the 

 Cherubin d' Orleans binocular would make an excellent ortho- 

 stereoscope with the addition of two carefully-worked Wheat- 

 stone's transposing prisms placed between the lenses of the 

 Huyghenian eye-piece. 



Tolles' binocular eye-piece is the same as Nachet I., with an 

 erector placed between the objective and the dividing prism. 



Wenham.| This excellent orthostereoscopic binocular passes 



* '* The non-inclining form alone is considered ; I have left out the 

 erecting prism, which only erects and does not transpose the image, as it 

 does not affect the stereoscopic conditions, and only introduces unnecessary 

 complications. 



t " Journal R. M. S.," 18.54, p. 20. See also 1892, p. 98. 



\ The development of this prism is interesting, and now almost forgotten. 

 The first arrangement was similar to that now known as Beck's dissecting 

 microscope, and when applied to the compound microscope gave pseudo- 

 stereoscopic pictures. It was also applied by Wenham above the eye- 

 piece ; this gave orthosteoroscopic images with a contracted field. The 

 second was an achromatic combination of three prisms not unlil^e an 

 achromatic concave cylindrical lens — there was transposition without a 

 cross-over ; it was, therefore, a pseudostereoscope. These date (1853) and 

 are figured in " R. M. S. Journal," 1854, p. 1. The third form was also an 

 achromatic combination of three prisms resembling an achromatic convex 

 cylindrical lens. In this case the transposition was corrected by a cross- 

 over; it was, tiierefore, orthosteoroscopic (" R. M. S. Journal," 1860, p. 155). 

 The fourth was the Wenham prism in its present form (" R. M. S. 

 Journal," 1861, p. 15). 



