4 Transactions of the Societ//. 



may be noted here that this disability obtains with all ordinary 

 opaque illuminators whenever prisms are employed.) Further, it 

 may be pointed out that the division must take place in the upper 

 focal plane of the objective, if a uniform illumination of the field 

 of vision is desired. This, however, becomes an impossibility with 

 dry lens systems of higher magnifying power, as in all systems 

 known to me the upper focal plane is situated within the lens, 

 where no mirrors or reflectors can be located, even if, as was done 

 in several English types, the objective mounts were made very 

 short. 



All these objections disappear with the physical method of 

 splitting up the pencils of rays, so that generally speaking this 

 method is considered the more advantageous one. The aperture 

 is not reduced, the field of vision is equally illuminated. There 

 are several types which make use of this method of division, viz. 

 (1) The binocular arrangement of Powell and Lealand,* where the 

 partial reflection from a thick glass plate is employed ; (2 ) The 

 so-called Wenham f-Schroder objective-prism, made by Eoss and 

 Co., of London ; and (3) the previously mentioned stereoscopic 

 eye-piece of Abbe. The last two named types divide the rays at 

 a thin film of air which transmits and reflects at the same time, 

 and this arrangement, just as with Powell and Lealand's type, 

 unavoidably causes a marked difference in the light intensity of 

 the two fields of vision. This difference, which amounts to a ratio 

 of about 1 : 2*5, with Abbe, and which is higher still with Powell 

 and Lealand, is under some circumstances even a desirable factor 

 for a stereoscopic effect such as is sought for in those types, while 

 for purely binocular examinations it is undesirable. In addition 

 to this it follows, at least with Abbe's arrangement, that two eye- 

 pieces of different construction have to be used, one a Huyghenian 

 and the other a Eamsden, and that only one degree of magnifica- 

 tion in the eye-piece is available. A further disadvantage of 

 Abbe's eye-piece is, that the two tubes are placed in a converging 

 position (see pp. 6, 7). 



3. The New Binoculak Miokoscope. 



The problem before us, therefore, was to construct a binocular 

 Microscope which can be used with any desired pair of eye-pieces, 

 in which the two fields of vision shall have equal intensity of 



* Described by L. Dippel, Das Mikrcskop und seine Anweudung. [2nd ed., 1882, 

 p. 55G. 



t F. Wenham, On a Binocular Microscope for High Powers. Trans. London 

 Micr. Soc, No. 14 (1866) pp. 103-6. Wenham himself does not appear to have 

 carried out this type of construction. In mentioning the Wenham-prism, with 

 English Microscopes, a different type, employing geometric division, is always 

 alluded to. 



