E. M. NELSON ON BINOCULAR MICROSCOPES. 373 



angle so that some of the light was passed while some was 

 reflected. As this took place at both surfaces a double image 

 was made in one tube, which, of course, was fatal to the design, 

 and the binocular never came into use. Prof. Abbe's binocular 

 eye-piece was made on a similar plan and failed for the same 

 reason. Subsequently, however, a method was discovered for 

 depositing a semi-translucent film of silver on glass, by which 

 means a beam could be half reflected and half transmitted. This 

 method was adopted by Ives, and the doubling of the image in the 

 one tube was avoided. The Ives binocular resembled the Wenham, 

 inasmuch as the prism could be withdrawn and the instrument 

 used as a monocular. But it also differed from it, for in the 

 Wenham the inter-ocular distance was adjusted by lengthening 

 or shortening the draw-tubes, while in the Ives it was accom- 

 plished by a lateral displacement of the side tube in arc, the 

 lower end of this tube being pivoted on a hinge. This was 

 a good design, for it permitted the inter-ocular distance to be 

 adjusted without disturbing the tube length. In 1860, when the 

 Wenham was first introduced, low powers, with their double 

 fronts, were very insensible to alteration of tube length, and as 

 all powers higher than a | had correction collars, any alteration 

 of tube length was of no moment ; this, however, no longer 

 applies, because objectives now made with single fronts having 

 over-corrected backs are very sensitive to tube length. So in 

 designing a binocular for use with such objectives, particular 

 attention must be given to tube-length adjustment. 



Now, lately, Messrs. Leitz have brought out a new binocular of 

 the Powell type ; the arrangement of the prisms, which deflect the 

 rays right and left, differs from the many kinds that have been 

 invented for this purpose. The semi- translucent silver film 

 method has been adopted by Messrs. Leitz in their new binocular, 

 and an almost equally illuminated image is seen in each tube. 

 By means of their very perfect system of working prisms they 

 have secured a really sharp critical image in each tube. The 

 tubes are parallel to one another, but the instrument cannot be 

 used as a monocular, for neither body is in the optic axis of the 

 objective. Messrs. Beck have also brought out a binocular 

 microscope with the two Ives prisms joined in one. The bodies 

 are converging, but as one body is in the optic axis of the 

 instrument, it can be used as a monocular. 



