62 



While the methods for transmitting the rays 

 from the objective to the binocular tube vary, the 

 construction in most common use is that introduced 

 by Mr. Wenham. 



By reference to .Fig. -25 it will be seen that the 

 rays from one-half the objective are transmitted 

 uninterruptedly to the vertical tube, while the 

 prism intercepts the rays from the other half and 

 by reflection induces them to pass into the oblique 

 tube. The result is an image in each eyepiece 

 thus giving stereoscopic vision. This gives a per- 

 ception of depth, a peculiar faculty of being able 

 to look into an object and conveys to the mind the 

 impression of roundness or separation of the 

 object into different planes which it is impossible 

 to obtain with monocular vision. Its use, how- 

 ever, is limited to the lower power objectives. 



Coarse Adjustment. In providing this adjust- 

 ment, two methods are followed. The most simple 

 form is that by a sliding tube in which the tube 

 which carries the nose-piece at the lower end and 

 draw-tube at the upper end, is moved up and down 

 in an outer sheath, which is fastened to the arm. 

 The milled ring, which is provided, is ^grasped by 

 thumb and fore and middle fingers and pushed 

 down or drawn up by a spiral motion. It is not to 

 be commended except for economical reasons, as 

 it lacks firmness, wears out quickly from the con- 

 siderable friction, endangers the object and the 



