64 HOW TO SEE WITH THE MICROSCOPE. 



use as an object finder, the safety nose-piece, the duplex 

 arrangement of sub-stage and mirror arm, whereby 

 these move in unison, or independently of each other r 

 and the centring and rotating sub-stage. 



To meet this, it might be claimed, on the part of the- 

 Centennial, that the above-named additional contri- 

 vances have been purposely discarded by the maker of 

 the Centennial ; that really there is no particular advan- 

 tages to be derived from the iris diaphragm, the mechan- 

 ical stage, the centring sub-stage, or the duplex 

 arrangement above referred to, and that all these con- 

 trivances serve unduly to complicate the instrument. 



As to the centring sub-stage, it might be also said 

 that with the introduction of the " swing" there is no 

 longer need of wide-angled achromatic condensers. 

 Furthermore the secondary body of the Centennial 

 being itself accurately centred, there is no occasion to 

 introduce especial appliances for this purpose. 



It will be noticed that in the Centennial the mirror 

 and sub-stage rise without hindrance above the stage, 

 the movement being only stopped by contact with the 

 body tube (supposing the smaller stage to be employed,) 

 all this time the mirror remaining in fixed focal posi- 

 tion. To accomplish the rise of the mirror above the 

 stage in Mr. Bulloch's stand, the jointed arms connected 

 with the mirror are brought into play, and indeed can- 

 not be dispensed with. A close comparison of the 

 mechanism will reveal the fact that in respect to the 

 swinging arrangement, there is a wide difference in 

 the construction of the two instruments. The angle of 



