18 1\ 5 of the Society. 



manner described by Dr. Jentzsch as "physically." The beam 

 of light is not bisected, but is as ir were sifted into two so that 

 - ne light from each portion of the beam goes to each eye. 



Powell and Zealand (Plate I, fig. 6). — The earliest type 

 will demonstrate this. The whole of the light impinges on the 

 first surface of the thick plate No. 1. and while some enters the 

 plate and passes through to form the direct image some is reflected 

 at the first surface, am night by prism N . 2 and reflected 



at an angle up the second tube of the Microscope. I have om 

 of these instruments, kindly lent by Mr. Muiron. 



7. The Wenham Modification of Powell and Zealand (Plate I, 

 fig. 7) cuts off the top of prism 1 and place- it below between 

 prisms 1 and 2, leaving an air space between prisms 1 and 2, and 

 slightly alters the angle of these prisms to get a larger proportion 

 of the light reflected. I have been unable to obtain a specimen 

 of one of these, and it is doubtful if any but experimental 

 instruments were made. 



8. The Abbe Binocular Eye-piece (Plate I, fig. 8) is optically 

 exactly the same as the Wenham -Powell, except that he divided 

 prism 2 and altered its angle so as not to cross the right- and 

 left-hand beams over. By his construction the length of path of 

 the two beams of light had been much altered. He used a 

 Huyghenian eye-piece on the left-hand body and a Eamsden eye- 

 piece on the right-hand body to correct this, as a Eamsden eye- 

 piece has its focus (F) at a much lower position than that of a 

 Huyghenian. Abbe, instead of using this instrument nearly over 

 the object-glass, placed it near the eye-piece. 



9. Dr. Jentzsch now describes the New Zeitz Binocular (Plate 1, 

 fig. 9), which makes use of a half-silvered him cemented between 

 two glass prisms to divide the beam. 



10. I have devised a still further form (Plate I, fhj. 10), which 

 I am exhibiting here to-night, which is somewhat on the lines 

 of the Wenham-Powell No. 6, but I am making use of the half- 

 silvered film for dividing the lisjlit, and am not crossing over the 

 beams for certain important mechanical reasons, and I also use a 

 parallel block of glass on one of the prisms for equalizing the 

 optical path of the two rays, so that both come to a focus at the 

 same position in the eye-piece. 



III. The third type of binocular Microscope consists of two 

 complete Microscopes pointed obliquely at the same object, and 

 is only useful for low powers. 



The Greenough Microscope, as made by Mr. C. P.aker, has 

 kindly been lent me bv Mr. Curties to illustrate this form. 



The paper by Dr. Jentzsch is a most admirable and interesting 

 comment on most of these instruments, and the qualities may 

 thus be summarized. 



