II. — The Binocular Microscope of the Past, and a New 

 Form of Instrument. 



By Conrad Beck. 



(Bead December 17, 1913.) 



Plates I, II. 



The paper by Dr. Jentzsch has suggested that an exhibition oi 

 the chief types of binocular Microscopes of the past and present 

 might be interesting. To illustrate the construction I show on 

 Plate I diagrams of the optical arrangement of the various forms. 

 Dr. Jentzsch has made a happy classification of binocular Micro- 

 scopes into two kinds, and, adopting this classification, I would 

 add a third. 



I. The first type of binocular Microscope is that in which 

 the light from a single object-glass is geometrically divided and 

 half directed into each eye. The beam of light is bisected. This 

 type includes in order of priority : 



1. llic Biddell (Plate I, fig. 1). — My example of this is a small 

 dissecting compound Microscope, made by Smith and Beck. 



2. The Achromatic Prism of Wenham and Biddell (Plate I, 

 fig. 2). — I can only obtain one of the prisms of this type, but I 

 have a simple achromatic Microscope on this priuciple made by 

 Smith and Beck. 



3. The Stephenson Prism (Plate I, fig. 3). — An example of this 

 has kindly been lent by Mr. Bousselet, and another by Mr. Curties. 



4. The Nachet (Plate I, fig. 4). — Sir Frank Crisp has kindly 

 lent us a sample of this form. 



5. The Wenham Binocular (Plate I, fig. 5). — I have two 

 examples of this, one in use with a low power and the other in 

 use with a high power mounted very close to the prism. I 

 have also an old experimental one, in which the prism is actually 

 mounted in the back cell of a ^ object-glass so as to be practically 

 in contact with the back lens, and another prism only mounted 

 on an arm which projects down into the interior of a high-power 

 object-glass. The Wenham form of binocular Microscope has been 

 sold in thousands and is the only binocular Microscope that has 

 had very great popularity. 



II. The second type of binocular Microscope is that in which 

 the light from a single object-glass is divided into two beams in a 



Feb. 18th, 191^ c 



