294 



ACCESSORY APPARATUS 



adapters to equalise the length of the objectives, so when a change 

 of objectives is made little change of focal adjustment is required. 

 Figs. 236, 237 show the nature of this arrangement. In Nelson's 

 changing nose-piece a small ring with three studs is screwed on to 

 the objective ; a nose-piece is screwed on the microscope, having 

 three slots and three inclined planes. Therefore, by placing the studs 

 into the slots and giving the objective a quarter of a turn, the 

 studs run up the inclined planes, thus causing the flanges to ' face 

 up ' tightly. 



Mr. Nelson has pointed out a far better and simpler method 

 which dispenses with all extra apparatus. 



Three portions of the thread in the nose-piece of the microscope 

 itself are cut away, and also three portions on the screw of the 



FIG. 236. Zeiss's sliding-objective 

 changer, with objective in position. 



FIG. 237. The objective detached from 

 the body-slide. 



objective. Those portions where the thread is left on the objective 

 pass through those spaces in the nose-piece where it has been cut 

 away. The screw engages just as if the whole screw were there, and 

 the objective faces up in the usual manner. This plan in no way 

 injures either the microscope or the objectives for use in the ordinary 

 way ; thus uncut objectives will screw into the nose-piece, and cut 

 objectives will screw into an uncut nose-piece. This plan is similar 

 to that employed in closing the breech of guns, and it was seeing one 

 of them in 1882 which suggested to Mr. Nelson to adapt the same 

 principle to the microscope. Subsequently it has been found that 

 in 1869 Mr. James Yogan had proposed much the same plan, only 

 cutting away two portions instead of three ; it is curious that such 

 an excellent idea was allowed to drop. 



An analysing nose-piece is that which carries a Nicol's analysing 



