86 BINOCULAR MICROSCOPES [Ch. Ill 



(Ives, Jour. Franklin Institute, Dec. 1902, pp. 441-445, fig. 53.) 

 See also the paper by Conrad Beck, with figures of the various 

 forms of dividing prisms of binoculars, Jour. Roy. Micr. Soc, 1914, 

 pp. 17-23 (fig. 54, 55). 



All single-objective binocular microscopes now on the market are 

 made in accordance with the principles enunciated in Mr. Ives' 

 original paper. 



§ 144. Parallel or converging tubes for binoculars. — As men- 

 tioned above, the original telescope-microscope, persisting in the 

 form of the opera-glass, had parallel tubes. Following the differentia- 

 tion of the telescope and microscope in which the objective of the micro- 

 scope gradually became of smaller diameter and shorter focus, and the 

 eye-piece of the original, concave Dutch form was replaced by the 

 convex, Keplerian form (see history at the end), the two objectives 

 of the binocular could be placed closer together, and in this way smaller 

 and smaller objects could be brought into the same field even with 

 quite high objectives. As the oculars must be separated sufficiently 

 to bring their axes in the middle of the pupil of the eye, the tubes must 

 be made more or less converging (fig. 52). 



The question is, which arrangement, parallel or converging tubes, 

 is easiest on the eyes of the observer for continuous work. The argu- 

 ment of those advocating the parallel arrangement is that when the 

 eyes are at rest, as in viewing distant objects, the rays entering the 

 eyes are practically parallel, and the two eye axes are of course also 

 parallel; and that with the most favorable focus of the microscope 

 the rays of light leaving the oculars are practically parallel, hence 

 the eyes should have their axes parallel as for viewing distant objects, 

 and that there is no effort at accommodation for getting the sharpest 

 image on the retina. 



For those who advocate converging tubes it is pointed out that in 

 observing small details the eye naturally uses the near point of distinct 

 vision, viz. 250 mm. for adults, and not the far point, and therefore 

 the most satisfactory microscopic work can be accomplished with 

 the converging tubes to correspond with the natural convergence 

 of the eyes. Much actual experience will doubtless be required for 

 the settlement of the question. From the author's experience with the 



