BATED & TATLOCK (LONDON) LTD. 



Double Demonstrating Eyepiece. 



'Ibis eyepiece enables two observers to view jointly an object under the microscope. It slips into the draw- 

 tube of the microscope like an ordinary eyepiece. The field of view is common to both eyepieces and contains 

 a pointer which either observer can direct upon any feature to which he wishes to draw attention. 



The arrangement of the device is shown in the subjoined figure : 



I. and II. are two prisms in contact and mounted above the diaphragm between the field lens and the eye lens of 

 the eyepiece. The prism I. has an isosceles cross section and its angles are 35, 35, and 110 respectively. 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 eyepiece 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 

 reflection. The centre line of the reflected pencil is inclined at an angle of 70 to the axis of the microscope. III. 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 a system of lenses which 

 resembles a terrestrial eyepiece. The image as seen in the side tube is reversed with respect to that which appeara 

 in the axial eyepiece ; but this would hardly affect the observer, especially since the oblique attachment of the 

 side eyepiece 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 eyepiece, 

 as the image seen through it is brighter and easier to focus. The adjustment for one eyepiece furnishes also a clearly 

 defined image in the subsidiary eyepiece, provided the eyes of both observers can accommodate in a similar manner. 

 The objective in conjunction with the filed lens below the double prism of the two eyepieces 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 subsidiary eyepieces. 



The pointer can be moved backwards and forwards and turns on a pivot so that its extreme end can be set to 

 any point in the field. The Double Demonstrating Eyepiece is made in two powers, one having a magnification of 

 4 diameters, the other of 6 diameters. In both cases the resulting images are sharp, colourless, and free from distor- 

 tion. The fact that the image seen in the subsidiary eyepiece is fainter than the other is no serious drawback 

 as the eyepieces are solely intended for demonstrating purposes, and the demonstrator's acquaintance with the 

 object will generally enable him to see every detail clearly under these less perfect conditions. When diffuse day- 

 light does not suffice to bring out fine details distinctly in the darker portions of the field it will be necessary to 

 use one of the artificial illuminators which are generally to be found in laboratories, such as electric glow-lamps, 

 arc lamps, Welsbach or acetylene lamps. With high power objectives it is generally advisable to set the draw-tube 

 of the microscope about i cm. shorter than its standard length. 



The Double Demonstrating Eyepiece is also well adapted for the instantaneous photography of living bacteria 

 and other moving organisms illuminated bv means of a dark-ground condenser. It enables one to watch the 

 object through the side eyepiece, and to defer the exposure until a favourable moment presents itself. 



1023 



1023' Large Mechanical Stage (Leitz's) for Stands A and B 



This stage is provided with finely cut racks, which render it available for searching preparations of 100 by 50 

 millimetres. The vernier for the advance and return motion reads i-io millimetre, that lor the lateral motion 

 1-20 millimetre. Attachment to the microscope is effected by placing the apparatus upon the microscope stage 

 with the knife edge at the back of the mechanical stage resting in the notch in the column of the stand, and 

 fixing by means of the milled thumb screw. This method ensures a rigid stage. 



This mechanical stage may be fitted at any time. 



CROSS STREET HATTO^ GARDETST, E- C 



