Wenham, on a Binocular and Single Microscope. 17 



Fig. 1. 



main tube. The additional body may either be soldered 

 permanently on to the other, or be made to draw on and off, 

 a double collar holding them 

 together at top, and a clip or bolt 

 at the bottom. In the latter case, 

 when the inclined tube is re- 

 moved, a cover should drop 

 neatly into the aperture, flush, 

 and be secured by a bolt. But 

 the additional body being no 

 hindrance to the ordinary action 

 of the microscope, it is best always 

 to allow it to remain in place ready 

 for instant use, as required. When 

 the prism is drawn out to its limit, 

 the main body acts just as the 

 usual single instrument, and 

 therefore needs no explanation ; 

 but on thrusting it in, a part of 

 the rays are thrown into the eye- 

 piece of the inclined body, and 

 thus the right-hand rays of the 

 object-glass are reflected into the 

 left eye, and the remainder pass 

 directly into the right eye, having 

 nothing intervening to obstruct 

 them in the due performance of 

 their best effect. The prism 

 need not, in all instances, be 

 thrust in to its fullest extent, so 

 as to take in the total half of the 

 object-glass, but only partially, 

 to the degree requisite for throw- 

 ing the object up in relief. In the case of a diflficult test 

 the largest share of the direct aperture may be employed, 

 while, by coaxing the illumination for the reflected portion, 

 the instrument can be made to perform well on the diato- 

 maceous tests. 



With respect to the illumination, in all cases where pos- 

 sible the opaque principle should be employed, as it gives 

 to objects a far more natural appearance. When transmitted 

 light is needed, a large, angular pencil should be used, other- 

 wise the two fields cannot be equally lighted with the higher 

 powers. I intend to have a split mirror made, each half 

 capable of separable and independent adjustment for each 

 body. The necessity for this will be shown with the Poclura, 



VOL. 1, NEW SER. b 



