38 



THE USE OF THE MICROSCOPE 



the two tubes meet on the ol)ject, and iDoth tubes are in 

 focus simultaneously; so that the two fields of view are 

 similar and equal in extent. This convergence (of 13 to 

 16 degrees) entails a parallel convergence of the eyes; which 

 is probably accompanied, more or less, by accommodation 



L R 



Fig. 11. — Diagram of the construction of the Greenough binocular, the angle 

 between objectives (and between eyepieces) being 15 degrees. The course of the 

 optic axis through the Porro prisms is indicated. The different object planes of the 

 two twin objectives are shown. The different positions of the lower and higher 

 power objectives are indicated. The interocular distance is varied by rotatmg 

 the two prism sets, which doea not alter the optical tube length. Note that 

 there are four total reflections on each side. (Periplane eyepieces are shown 

 here.) 



to suit. The distance of the point of convergence for the 

 eyes is 8>^ to 10 inches. (Some makes of Greenough have 

 the objectives converging more than the eyepieces, probably 

 through obliquity of prisms.) 



