PRACTICAL MICROSCOPY. 



than with a monocular microscope. Binocular instruments 

 are made in such a manner that the rays proceeding from 

 an object lying upon the stage are split into two parts, 

 each portion passing separately through a tube to the 

 eye-pieces, as shown in Fig. 17. 



The form of prism used for dividing the rays (we may say 

 universally) is that devised by Mr. Wenham many years 

 ago. It is mounted in a small brass box, sliding into the 

 lower end of the microscope body immediately over the 



objective, as shown in Fig. 17. 

 A section of this prism is shown 

 in Fig. 1 8, the dotted line indi- 

 cating the direction of one por- 

 tion of the divided ray. 



It should not be forgotten 

 that a binocular of this form is 



FIG. 17. 



FIG. 18. 



not well suited for use with objectives of a higher numerical 

 aperture than o 34 or 40 air angle ; a certain amount of dis- 

 tortion occurs with higher apertures, which may be readily 

 perceived on the examination of spherical pollen-grains and 

 such like objects. Dr. Carpenter has pointed out this ex- 

 aggerated relief years ago, yet there are some folks now 



