The Binocular Microscope of the Past, and a New Form. 23 



a touch and either binocular vision with half light in each eye or 

 monocular vision with full light in one can be employed as desired. 



The question of slightly convergent versus parallel tubes for a 

 binocular Microscope is one which depends on whether the observer 

 who uses the Microscope uses his accommodation or not. It is, of 

 course, well known to oculists that convergence and accommoda- 

 tion act together, not that they are actually interconnected, but 

 that by long habit when the accommodation is used the eyes con- 

 verge, and vice versa, and if when the accommodation is used the 

 eyes are obliged to look out parallel, considerable eye-strain and 

 fatigue are occasioned. For telescopes, which are used when the 

 observer is viewing objects at a distance, the eyes are naturally 

 looking out parallel, and parallel tubes should be used, but where 

 the Microscope is employed, alternately with the examination of 

 objects on the table along side it, at a distance of say 12 or 15 in. 

 the observer's eyes are converged to this distance, and it would ap- 

 pear reasonable that such would be the angle at which the Micro- 

 scope tubes should be set. 



A further advantage of this method of constructing the tubes is 

 that by a slight movement of the head backwards or forwards the 

 observer can obtain stereoscopic, flat, or pseudoscopic vision. The 

 distance apart of the eye-pieces of the Microscope is varied by 

 rotating one tube with the finger and thumb. They should be set 

 at such a distance apart that the complete Eamsden disk of each 

 eye-piece is central with the pupil of each eye. Under these cir- 

 cumstances the full resolution and no stereoscopic relief is obtained. 

 Xow if the eyes be placed a little closer to the eye-piece the 

 observer's pupils cut off the margins of the Eamsden disks and the 

 stereoscopic effect is obtained ; on the other hand, if the eyes are 

 drawn back a pseudoscopic image is seen. Under the Microscope 

 which is on exhibition will be seen a number of Coscinodiscus 

 showing the spines whose composition created discussion some 

 time back m the Society. When the observer examines these, 

 moving his eyes towards or from the Microscope, a pair of individ- 

 ual specimens, which are on different planes will completely change 

 over their relative positions, at one time a specimen being behind, 

 at another time in front of its neighbour. Thus an observer using 

 this instrument for resolution under the most perfect conditions can 

 instantly obtain the perception of the relative depth of the parts 

 by a slight movement of the head. 



in working with this Microscope so far I have found no disad- 

 vantage as compared with a monocular instrument, but even if 

 such should sometimes exist, a touch slides the prism out of position 

 and it is then exactly the same in every respect as the ordinary 

 monocular Microscope. 



