36 PEACTICAL PHOTO-MICKOGKAPHY 



constructed that the objectives it carries would, when brought 

 into the position of the optical axis, be always equally well 

 centred. The consequence is, as already mentioned, that if the 

 object is centred with one objective, say a low-power one, and 

 then a high power is brought into use, the object is not in the 

 centre of the field, and in some cases the want of centration is 

 so great that the image may actually be thrown out of the 

 field altogether. 



Some makers are giving greater attention to these appliances, 

 and the results obtained are better than formerly ; but it is still 

 rare for a triple or quadruple nose-piece to be so accurately 

 made that all the objectives which it carries are equally well 

 centred. Messrs. J. Swift & Son have, however, recently 

 produced some nose-pieces that are extremely accurately 

 made and at least, when they leave their hands admit of 

 little improvement in this respect. 



Another objection to these multiple-changers is that a 

 considerable additional weight consisting of two, three, or four 

 objectives as well as the changer itself is thrown on to the body- 

 tube of the microscope ; and as this weight is thrown only on 

 one side it is a very real objection in critical work. A further 

 disadvantage is that it is very difficult to ensure that the optical 

 axis of the objectives are in exact alignment with the optical axis 

 of the microscope, and should this occur with a lens of large 

 aperture it would seriously interfere with its performance. It is 

 far better, if possible, for photo-micrographic work to do with- 

 out such a method of changing, as though it may be convenient 

 there is too great a possibility of introducing errors in the 

 resulting photograph. 



An arrangement that is not open to any of these objections 

 is the sliding objective-changer made by Messrs. Zeiss (Fig. 10). 

 In these a slide is screwed on to the microscope in the position 

 usually occupied by the objective, and it is so arranged that 

 an objective may be inserted from any direction. This slide 

 becomes a permanent attachment to the microscope body-tube. 

 On each objective a counterpart of this is screwed, so that it is 

 only necessary to slide one portion of the changer into the 

 other to change any objective. There is an arrangement 

 on each changer for centreing the objective accurately to 

 the optic axis of the microscope, and this adjustment should 



