370 E. M. NELSON ON BINOCULAR MICROSCOPES. 



four attributes of binocular vision are secured. In tins micro- 

 scops the left-hand view of the objective is sent into the left 

 eye, and the right-hand view into the right eye; this, because 

 of the erection of the image, gives an ortho-stereoscopic image. 

 If the microscope had been of the ordinary inverting type the 

 image would have been pseudo-stereoscopic. It was due to 

 ignorance of this principle that several of the early bino- 

 culars were pseudo-stereoscopes. One of the most important 

 points in this, as well as in all forms of binoculars, is that 

 the images should be accurately superimposed. Several tests 

 have been proposed ; one was that an object should be 

 placed upon the stage, so that it should just touch, say, the 

 right edge of the field of the right-hand eye-piece. This eye- 

 piece is then transferred to the left-hand tube, and if the object 

 still touches the same portion of the field with the same eye- 

 piece the adjustment was supposed to be correct. But this 

 is no test at all, for it tells you nothing about the really 

 important question, which is whether the discs of the fields 

 are themselves superimposed. 



The best test for a Greenough is to oscillate rapidly a strip 

 of card half-inch wide before the fronts of the objectives. If 

 the images shake, then they are not accurately superimposed, 

 and the objectives require readjusting in their seats. 



Leaving now the twin microscope, we will pass on to the 

 other kind of binocular, which has only one objective. In the 

 Wenham this important adjustment is performed by the align- 

 ment of the tubes, for the tilt of the prism has very little 

 effect, but its edge must be carefully set at right angles to 

 a line joining the centres of the eye-pieces. 



The single objective binocular may be divided into two kinds, 

 viz. those of the Wenham or Stephenson type, which split the 

 beam at the back of the objective, and those of the Fowell type, 

 which pass the whole beam. All those of the Wenham type 

 possess the first of the attributes enumerated above, viz. stereo- 

 scopic effect, for in an ordinary inverting microscope, at the 

 left-hand eye-piece the Ramsden disc will be a miniature of 

 a cross-section of the beam issuing from the right-hand half 

 of the objective, and that at the right-hand eye-piece from the 

 left-hand half of the objective, the inversion of the image 

 necessitating a cross-over of the pencils, for if there were no 



