USE AND MANIPULATION OF MICEOSCOPE 163 



over this difficulty, as in effect they enlarge the image of the 

 source of light. If for any reason the only method of getting 

 even illumination is by altering the position of the condenser 

 that is, by departing from the conditions necessary to obtain 

 critical illumination then it would appear to be better for the 

 condenser to be raised nearer the object-slide, so that the apex 

 of the cone of illumination is above the object, rather than 

 for the reverse operation to be performed. 



It should be remembered that sub-stage condensers are 

 always of very short focus, and it is therefore advisable that 

 in making these adjustments only medium-power objectives 

 should be on the microscope, so that in the event of the 

 condenser being raised too high and coming into contact with 

 the underside of the object-slide, the latter is not forced up 

 against the front lens of the objective. It may sometimes 

 happen with a very thick slide that it is impossible to focus 

 correctly on the object, and in this case, of course, critical 

 illumination is unobtainable. This condition may be to some 

 slight extent overcome by bringing the illuminant nearer the 

 condenser ; but, obviously, such a method is but a makeshift and 

 is a remedy which can only be effectual within narrow limits. 



It must not be overlooked that when using any form of 

 objective-changer such as a multiple nose-piece the sub- 

 stage condenser must be re-centred for each change of objective. 

 It very rarely happens that these appliances are sufficiently 

 correctly made to ensure the optical axis of a series of objectives 

 always coming in the same position. With the Zeiss objective- 

 changer or any similar appliance this does not apply, as it is 

 usual for the worker to centre carefully the whole series of 

 objectives in use when the changers are fitted to the objective. 



In centreing, where a change of objective becomes necessary, 

 the following method is satisfactory. Place on the microscope the 

 objective with which the photograph is to be taken, presumably 

 a high-power one. The object on the stage may conveniently 

 be a glass slip on which two diamond lines or scratches at 

 right angles have been made, as already described, this being 

 the most convenient object to use for all preliminary centre- 

 ing purposes. Arrange this object on the stage so that the 

 point of intersection of the diamond scratches appears exactly 

 in the centre of the field. Now remove the high-power objective 



M2 



