E. M. NELSON ON MICROSCOPICAL OBJECTIVES. 153 



The alteration will not interfere with the interchangeability of 

 the parts so altered with those that have not been altered. 



No new apparatus is required. 



The centering* of the objectives is not in the least disturbed. 



The change can be made with nearly the same rapidity as 

 with the double or triple nose-piece, the objectionable weight of 

 which is avoided. 



The rapidity of fixing is of use in renewing the fluid when work- 

 ing with immersion objectives. 



DISADVANTAGES. 



It will not be interchangeable because the existing arrange- 

 ments are not interchangeable. 



The objectives must be inserted in the nose-piece in one position. 



Care must be exercised in turning a stiff adjustment collar in a 

 direction that will unscrew the objective. 



[Note. — This is a disadvantage which to a great extent applies to 

 existing arrangements, for screw collars are only fitted to high 

 powers and wide angled lenses which have short-working distances. 

 If care, therefore, were not taken in turning a stiff adjustment 

 collar in a direction that would unscrew the lens, it would soon be 

 screwed down on the cover glass.] 



Persons who are unskilful in handling mechanical appliances will 

 perhaps find some trouble in acquiring facility in using it. 



