Transactions of the Royal Mieroscojncal Society. 113 



IV. — 0)1 a New Form of Binocular Eye-piece and Binocular 



Microscope for High Poivers. By C. D. Ahkens. 



{Taken as read before the Eoyal Microscopical Society, Feb. 8.) 



It is now about four years ago since I made a binocular microscope, 

 tlie prism of which was made of Iceland spar. It was not made 

 public at the time through any of the journals, as I could not get 

 it to define so perfectly as I wished. There werfe, however, several 

 gentlemen to whom I showed it at the time, and explained its con- 

 struction ; among these were Dr. Millar, Mr. Wenham, and several 

 others. Mr. Wenham then referred to his having tried to make the 

 same some years previously, and said that I had been more success- 

 ful than he was. By means of much labour and perseverance I 

 have, since the time referred to, brought the idea to a successful 

 issue, one which at starting I had scarcely contemplated. 



Through having been called upon to make many hundreds of 

 prisms for the beautiful arrangement by Mr. Wenham, the subject 

 of binocular vision is one in which I feel an especial degree of in- 

 terest, and in consequence I have been led to try if there were not 

 some other combination that could be made, by means of which we 

 might use the high powers as well as the lower ones, and still have 

 a stereoscopic effect, both fields being equally and evenly illumi- 

 nated without any extra or special illumination. I believe that I 

 have now succeeded in doing so. 



The means of effecting this is, as I said, obtained through the 

 agency of Iceland spar. I make a double-image prism out of spar 

 alone, for I get just double the amount of separation of the rays 

 that I should from one of glass and spar. I cut the spar in a way 

 that would be somewhat difficult for me to explain satisfactorily. 

 I grind one piece to an angle of about 40'^, and then cut it in two, 

 and reverse them, placing the thickest parts opposite each other, and 

 cementing them together with balsam. At the same time I cement 

 in a similar manner a piece of thin parallel glass on each end, to 

 preserve the surface ; and I find that by this addition I also get 

 better definition. In each of these pieces of spar there are two 

 images, the ordinary and the extraordinaiy rays ; and when these 

 two pieces are balsamed together we can only see two, that is the 

 extraordinary rays that I make use of, the two ordinary ones 

 being shut off; and I get two perfect uniform images of the same 

 separation out of one piece of spar. This prism divides the rays 

 about 2|- inches in 8 inches, which I find very convenient for the 

 microscope, allowing rack-work for the different widths of sights. 

 Of course there is a great deal of colour in each of these rays, 

 but I do away with this by placing a double-wedge flint glass 

 prism of an angle of 25^ (as sho-\vn in the drawing) over the 

 double-image prisms, and as close as I can get them just to allow 

 the fields to be illuminated aU over; and these prisms cause the 



