22 Transaction* of the Society. 



observer's eyelashes it cannot be placed nearly at the correct 

 position, and consequently many observers denied that any stereo- 

 scopic relief was obtained with his binocular eye-piece. The use 

 of his eye-piece diaphragms with any but very low power eye- 

 pieces is unsatisfactory, because the Eamsden disk is very close to 

 the Microscope, except in the case of eye-pieces of either very low 

 power or of special construction. But there is another way of 

 stopping out the portions required to give a stereoscopic picture, 

 and that is by placing the eye-pieces slightly too near or too far 

 apart, so that the pupils of the observer's eyes form the necessary 

 diaphragms to cut down the Eamsden disk ; and as the stereoscopic 

 effect Avith a high power object glass is generally exaggerated, a 

 very slight movement of the eye-pieces is often sufficient to cause 

 the necessary stereoscopic effect, and very little loss of light is 

 occasioned. 



5. In Types (5), (7), (8), and (10) the two eyes are converged to 

 a point generally about 12 or 15 in. from the observer. Dr. Jentzsch 

 states that to ensure comfort the eye should be looking out parallel. 

 This is not borne out by experience, as the Wenham binocular of 

 the ordinary form gives perfect rest to the eyes when used for 

 hours at a time, and I am inclined to expect it will always be 

 found more comfortable to use an instrument with the eyes con- 

 verging at an angle of about 10° to 15°. One thing is certain, that 

 the convergence must not be more than 18° and probably not so 

 great, as this is an angle to which the observer is not accustomed. 

 It will be noticed that with the Wenham binocular (Form 5), and 

 the Powell and Lealand (Form 6), if the angle of convergence of 

 the bodies is not to exceed the above convenient amount, the tubes 

 must be long in order that the eye-pieces may be at the required 

 distance apart. It is this fact, together with the trouble of illu- 

 mination and loss of resolution, which has prevented the Wenham 

 and Powell and Lealand (Forms Nos. 5 and 6) from being even 

 more extensively used than has been the case. It is since the 

 time of short tube-lengths that the popularity of binocular Micro- 

 scopes has ceased to advance. Any binocular Microscope that is 

 to be universal must have all the advantages of the monocular. In 

 my design of binocular Microscope which I am showing for the 

 first time to-night, I have been able to retain the small convergent 

 ansle of about 14° for the two tubes and yet use a tube-length so 

 short that even with a triple nose-piece attached it does not exceed 

 the 160 mm. standard short tube-length. You will notice that the 

 apex of the triangle at which the optic axes of the tubes meet is 

 3£ in. below the prisms (see Plate II). This advantage is gained 

 by altering the construction of the Wenham Powell model so that 

 the rays do not cross over when deflected from the prisms into the 

 tubes. It will also be noticed that if for certain purposes monocular 

 vision is required, the prism may be pushed out of position with 



