NACHET S STEREOSCOPIC BINOCULAR. 



31 



' crossing ' of the Pictures in the Stereoscope, or by that reversal of 

 the two perspective projections formed direct from the Object which 

 is effected by the Pseudoscope (§ 24). From waut of a due appre- 

 ciation of this principle (the truth of which can now be practically 

 demonstrated, § 29), the earlier attempts at producing a Stereoscopic 

 Binocular Microscope tended rather to produce a ' Pseudoscopic 

 conversion ' of the objects viewed by it, than to represent them in 

 their true relief. 



26. Nacket's Stereoscopic Binocular. — The first really satisfac- 

 tory solution of the problem was that worked out by MM. Nachet ; 

 whose original Binocular was constructed on the method shown in 

 Fig. 17. The cone of rays issuing from the upper end of the Objec- 

 tive meets the flat surface of a Prism p whose section is an equi- 

 lateral triangle ; and is divided by reflexion within this prism 

 into two lateral halves, which cross each other in its interior. For 

 the ray's of a b forming the right half of the cone, impinging very 

 obliquely on the internal face of the prism, suffer total reflexion 

 (§ 2), emerging through its left side at right angles to its surface, 



and therefore un- 

 • FlG# 17, dergoing no re- 



fraction ; whilst 

 the rays a' b' 

 forming the left 

 half of the cone, 

 are reflected in 

 like manner to- 

 wards the right. 

 Each of these 

 pencils is re- 

 ceived by a late- 

 ral Prism, which 

 again changes its 

 direction, so as to 

 render it parallel 

 to its original 

 course ; and thus 

 the two halves 

 a b and a' b' of 

 the original pen- 

 cil are complete- 

 ly separated from 

 each other, the 

 former being re- 

 ceived into the 

 left-hand body of the Microscope (Fig. 18), and the latter 

 into its right-hand body. These two bodies are parallel ; and 

 by means of an adjusting screw at their base, which alters the 

 distance between the central and the lateral Prisms, they can be 



Arrangement of Prisms in Nachet's Stereoscopic 

 Binocular Microscope. 



