432 EDWARD M. NELSON ON 



the eye-pieces for producing stereoscopic \'ision is due to Wen- 

 ham. There are those who are unable to combine the images, 

 even in a Greenough, but who have no difficulty in doing so in 

 a Parallel tube binocular ; while the experience of others is just 

 the reverse. It is probable that less than half those who use 

 microscopes are able to see a stereoscopic image in a microscope, 

 or in any other optical instrument. With regard to the sharpness 

 of the images, those in the Greenough and Parallel tube were very 

 similar, and both were sharper than those in the Wenham. The 

 difference in the apparent sizes of the images in those three 

 binoculars is due to the fact that the apparent distances of the 

 projected pictures are different. With the Parallel tube binocular 

 the eye sees the virtual images projected to a greater distance 

 than with the convergent tubes of the Greenough. It has been 

 stated that there is no definite measurable distance for the 

 plane of the virtual image, but obviously these experiments 

 prove that with a binocular it is not so. 



As 76 is the highest power with my Greenough, the following 

 experiments will be confined to the other two binoculars. The 

 object used in the previous experiments was a common circular, 

 and not quite spherical, polycistine, mounted opaque, and illumin- 

 ated by a side reflector ; but as we are now about to deal with. 

 higher powers, a more suitable object must be used. A good one 

 is a diatom, formerly called Actinosfhenia splendens, but now 

 known as Actinoptycus. Strictly speaking, it is an Aulaco- 

 discus, it has ten convex rays, down the centre of each runs a 

 true " aulax," ending in a small process. When this is examined 

 with, say, a 1-inch objective on a dark ground with a condenser of 

 low angle, or a spot lens, the aulax is seen as a bright white line, 

 but with a condenser of larger angle it becomes a black line. 

 The reason for this is not understood. 



This aulax is, by the way, an excellent test when viewed upon 

 a dark ground for medium objectives, say 2/3 and 1/2. When 

 this diatom was placed under the Wenham, power 270 and dark 

 ground, the aulax when in a horizontal position was sharply 

 defined, but when in a vertical one it was poorly imaged. In 

 the Parallel tube instrument, under the same power, the aulax 

 was sharply defined in all directions. This test is a good one 



