158 A MANUAL OF BACTERIOLOGY 



observations, the objective should be screwed directly 

 on to the tube unless Zeiss objective changers are employed. 

 These consist of a special female slide attached to the 

 tube, and a male slide to fit it attached to each of the 

 objectives. By means of two screws working at right 

 angles to each other on the male slides each objective 

 may be accurately centred once and for all. The Sloan 

 objective changer is another and cheaper device to effect 

 the same end (manufactured by Messrs. Beck). In order 

 to centre the objective changers, select any object having 

 some feature plainly visible with the lowest power, e.g. a 

 tissue section with cross-section of a small vessel. Attach 

 the lowest available objective directly to the tube of the 

 microscope and adjust the object in the centre of the field. 

 Unscrew the objective and attach the tube-slide to the 

 microscope. Screw the same objective to its sliding 

 changer and place on the microscope. Now view the 

 object, and by means of the two centring screws bring 

 the image to the centre of the field ; the objective will 

 now be centred to the axis of the body tube. The same 

 procedure is carried out with the other objectives, each 

 of which has its own sliding changer, which is left attached 

 to the objective. 



The magnifying power of the optical system may be 

 arrived at in the following way. It may be taken that 

 a 1-in. objective with a tube length of 250 mm. has an 

 initial magnification of 10 diameters ; if the tube length 



170 

 be 170 mm., the magnification is - X 10 = 7 nearly. 



With a |-in. lens, the magnification = X 10 X - =14, 



ZoU 1 



and so on. The initial magnification of the lens is, how- 

 ever, also magnified by the ocular ; if the ocular be a x 4, 

 the final magnification with the 1-in. objective and 170- 

 mm. tube would be 28 diameters. 



