164 PEACTICAL PHOTO-MICEOGEAPHY 



and substitute for it one of medium power, and carry out the 

 necessary operations to obtain correct centration, as already 

 described ; but, in this case, the illuminant and condenser 

 must be centrated not to the centre of the visible field of the 

 objective, but to the point where the images of the diamond 

 lines intersect, since this is the position of the optical centre 

 of the objective to be used. The centration will therefore be 

 correct when the high-power objective is replaced with which 

 the photograph is to be taken. Assuming that the illuminant 

 and any auxiliary condensing-lens have been carefully centred for 

 one objective, it is usually not necessary to alter the position 

 of these when effecting a change of objectives ; or if any 

 movement is necessary it is very slight. The differences of 

 centration in the latter are usually not so great as to render it 

 necessary to alter the position of the illuminant, re-centration 

 of the sub-stage condenser being sufficient to effect the desired 

 result. 



It is assumed that the adjustments so far described have been 

 made, or at least that there has been some preliminary practice 

 of the process with a microscope on a bench and not on the 

 photo-micrographic apparatus. The centration of the object 

 and the sub-stage condenser having been effected, it remains 

 to place the microscope in a horizontal position on the photo- 

 graphic apparatus, and the illuminant in position at a suitable 

 distance from the microscope. If no auxiliary condenser or 

 bull's-eye is used, this distance will be about ten inches. It 

 will then probably be found that the image of the illuminant is 

 not in the centre of the field. To watch this image through 

 the microscope and alter the position of the illuminant so that 

 it is exactly central is usually a somewhat troublesome process, 

 as it is generally difficult to reach the source of light while 

 observing the microscopic image. It is assumed that the 

 microscope is carried on a revolving table, so that it can be 

 thrown out of the optical axis or that the camera itself may 

 be moved back to allow of direct observation of the image. 

 If the apparatus is of the former type, allowing of the 

 movement of the microscope and illuminant as a whole, the 

 great advantage of the small subsidiary opaque screen, set 

 at the side of the focussing-screen of the camera as already 

 described, will now become apparent. It makes it possible 



