POWELL AND LEAL AND S MICROSCOPE. 



Ill 



on 



the. 



Fio. 28. 



axis, instead of 



vibration. The stage is provided witli a traversing movement 

 in each direction, to the extent of about three-quarters of an 

 inch ; this is effected on the plan known as Turrell's, in which 

 the two milled heads 

 are placed 

 same 



side by side, one of 

 them being also re- 

 peated on the left 

 hand of the stage, so 

 that the movements 

 may be communi- 

 cated either by the 

 right hand alone, or 

 by both hands in com- 

 bination. The plat- 

 form which carries 

 the object is made to 

 slide, as in the preced- 

 ing case, on the sum- 

 mit of the traversing 

 apparatus; and it has 

 not only a ledge 

 whereon the object 

 may rest, but also a 

 " spring clip" for se- 

 curing it whenever 

 the stage may be 

 placed in a vertical 

 position. This plat- 

 form, moreover, is so 

 connected with the 

 traversing apparatus, 

 that it may be turned 

 round in the direction 

 of its plane ; but as 

 this rotation takes place abo-se instead of beneath the traversing 

 apparatus, there is no security that the centre of rotation shall 

 coincide with the axis of the optical portion of the instrument ; so 

 that, unless this adjustment have been previously made, the object 

 will be thrown completely out of the field of view when the 

 platform is made to revolve. Hence, although this movement is 

 of great use in facilitating the full examination of an object, 

 by enabling the observer to bring it into the field of view in 

 every variety of position, it does not serve, like the rotatory 

 movement of Mr. Ross's stage, to change the position of the 

 object in regard to the illuminating apparatus, vdthout disturb- 

 ing the observer's view of it. The condenser for transparent 

 objects,, the polarizing apparatus, &c., are here fitted to the under 



Powell and Lealand's Large Compound Microscope. 



