THE 



MONTHLY MICROSCOPICAL JOURNAL. 



JUNE 1, 1870. 



I. — The New Binocular Microscope. By Samuel Holmes. 



(Read be/ore the Koyal Microscopical Society, May 11, 1870.) 



Plate LIU. 



My microscopes are arranged to be Monocular, Binocular, Pseudo- 

 scopic, or Stereoscopic, at the pleasure of the observer. 



The definition and illumiuation are equal in both fields of view. 

 They give pictures in sohd relief of Opaque objects, and show the 

 thickness of the structure in Transparent objects. 



There have been several forms of Binocular Microscope abeady 

 contrived, but an examination of the principles of their construction 

 shows that much has been left to future endeavour. 



In the binocular microscope of Nachet, since, I beheve, some- 

 what modified, we have several extra surfaces of glass for the light 

 to pass through in the necessary prisms, and u-respective of the 

 difficulty of ensuring correct internal transmission through these 

 prisms, their surfaces reflect and scatter the Hght that a more 

 scientific combination might conserve. 



Mr. Wenham's first scheme, of two achromatic prisms over the 

 object-glass, was open to the same objection, and had a similar 

 defect of increased surface although in a less degree, and the diffi- 

 culty of execution was great from the thinness and acuteness of the 

 prisms. 



In Mr. Wenham's second plan, smce so deservedly popular, the 

 prism requires nice execution, and depends for the similarity of 

 definition of its picture, on the equal transmission of hght through 

 a thicJc piece of glass, and on the goodness of its fom- surfaces. 



These difficulties, combined with the error acquired by the 

 unequal length of one set of rays over that of the other set, render 

 it almost impossible to make an instrument on this plan in which 

 the errors are not perceivable by the eye ; and thus, although the 

 illusion of solidity is presented to the observer, a doubt has arisen 

 as to whether it has been properly obtained and is to be trusted. 



The most perfect piece of glass is variable in structure, and 

 afiects the passage of hght through it in an irregular manner. 

 This may be called structural aberration, and is reduced to its least 



VOL. III. T 



