448 



SUMMAKY OF CUKRENT KESE ARCHES RELATING TO 



The arrangement of the device is shown in the subjoined figure 

 (fig. 78). 



I and II are two prisms in contact and mounted above the diaphragm 

 between the field-lens and the eye-lens of the eye-piece. The prism I has 

 an isosceles cross section, and its angles are 35°, 35° and 11(J° respect- 

 ively. The prism II is rectangular, and its angles are 35°, 55° and 90°. 

 The prisms are placed with those faces in contact which subtend the 

 angles of 90° and 110° in such a manner as to leave between them a very 

 thin film of air. This film is inclined at an angle of 30° to the axis of 

 the eye-piece and partially reflects the emerging pencil of rays ; about 

 two-thirds of the rays pass through the prisms and one-third is reflected. 



The image formed along the axis of the Microscope is accordingly 

 brighter than that produced by partial reflexion. The centre line of the 

 reflected pencil is inclined at an angle of 70° to the axis of the Micro- 

 scope. Ill is the prism, the lower surface of which reflects the pencil 

 upwards at a convenient angle for observation. In order that the two 

 observers may not be in each other's way the branch-tube is fitted with 



Fig. 78. 



a system of lenses which resembles a terrestrial eye-piece. The image 

 as seen in the side tube is reversed with respect to that which appears in 

 the axial eye-piece ; but this would hardly affect the observer, especially 

 since the ol)lique attachment of the side eye-piece already introduces 

 unusual conditions of working. 



As a matter of fact the more expedient course is to adjust and focus 

 the object through the principal eye-piece, as the image seen through it 

 is brighter and easier to focus. The adjustment for one eye-piece fur- 

 nishes also a clearly defined image in the subsidiary eye-piece, provided 

 the eyes of both observers can accommodate in a similar manner. The 

 objective in conjunction with the field lens below the double prisms of 

 the two eye-pieces forms an image in the plane of the diaphragm below 

 the double prism. This image and the pointer being both in the plane 

 of the diaphragm are seen simultaneously in the principal and the sub- 

 sidiary eye-piece. 



The pointer can be moved backwards and forwards and turns on a 

 pivot so that its extreme end can be set at any point in the field. The 

 Double Demonstrating Eye-piece is made in two powers, one having a 

 magnification of four diameters and the other of six diameters. 



This eye-piece is also well adapted for the instantaneous photography 

 of living bacteria and other moving organisms illuminated by means of 



