CHAPTER III 

 THE TWIN -OBJECTIVE BINOCULAR 



Low-power Microscopes. — Low-power microscopes may 

 include the following instruments: (1) The low-power 

 monocular microscope, and the low-power monobjective 

 binocular, used with obj-ectives up to 0.2 or 0.3 aperture. 

 Both of these reverse the image. (2) The twin-objective 

 binocular, with objectives up to about 0.1 aperture; the 

 Lihotzky binocular attachment; and the proposed micro- 

 scope consisting of Porro prisms combined with a half- 

 silvered Swan cube as a monobjective binocular. These 

 give an erect image. Different binoculars have either 

 a constant or an alterable stereoscopic effect. Erect 

 images, binocular \4sion, and some stereoscopic effect 

 are useful in a low-power microscope. The twin-objective 

 (Greenough) binocular can only be used with low objectives, 

 while the other forms can be used with medium or high 

 powers. They will be described later. The following is a 

 comparison between the twin-objective binocular and the 

 monobjective binocular: 



Twin-objective Binocular Monobjective Binocular 



1. Erect images. Erect images only if Porro prisms, 



or erecting lenses, are used. 



2. Permanent stereoscopic effect. Alterable stereoscopic effect. 



3. Converging tubes. Parallel tubes, usually. 



4. Special objectives. Ordinary objectives. 



5. Aperture up to about 0.15. Any aperture. 



6. Fields of view inclined about 1.5 Fields of view coincident in plane. 



degrees to each other. 



Convergence of Tubes. — In the twin-objective binocular 

 (Fig. 11), the two microscope tubes (each furnished with a 

 compact set of Porro erecting prisms) are so inclined to 

 one another that, when in correct adjustment, the axes of 



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