ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



577 



or vice-versa. It is supplied in two forms : (1) attachable ; (2) built-in. 

 The attachable form is shown separately in fig. 56, and in position in 

 fig. 57, and is screwed to the objective-glass end of a monocular body 

 in precisely the same way as an ordinary revolving nose-piece is fitted. 

 It can, therefore, be placed on the instrument required, and thus convert 

 a monocular into a binocular. The built-in form is a complete binocular 

 body, and is provided with a sliding carrier for the prism, whereby the 

 prism can be withdrawn from the field and the light, therefore, enabled 



dlt U f''!'K:i 



Fig. 57. 



to pass directly up the monocular body only, without obstruction, when 

 so desired. The binocular thus becomes a monocular. Both forms 

 depend upon an optical system, first employed by Abbe, for dividing the 

 light coming from the objective, the dividing prism being placed close 

 to the objective, and the lateral prism being provided with a glass 

 extension of such length as to cause the two separated beams to focus 

 at the same distance from the objective. The instrument also includes 

 an ingenious arrangement for adjustment of width of eyes without 

 alteration of tube length. The lateral prism and the tube carrying 



Dec. 16th, 1914- 2 q 



