128 



Under these circumstances, it appeared to me that these difficulties 

 might be overcome by a different arrangement of the parts of the stage, 

 without at the same time detracting in any great degree from the con- 

 veniences and appliances existing in the stages at present employed ; 

 this I propose to effect by changing the position of the revolving- 

 plate, and placing it below instead of above the actions of the stage ; 

 by adopting this principle it is evident that if the centre of the 

 revoloing-plate be a fixed point coinciding exactly with the axis of 

 the microscope, the object under examination must remain in the field 

 of view during an entire revolution, however eccentric the adjustments 

 may be : in confirmation of this principle I was induced to construct 

 the rough model which you see on the table, and my views have since 

 been admirably carried out by Messrs. Smith and Beck, in adapting 

 a new stage to my instrument. 



The principal feature then, in this improvement, consists in fitting 

 the revolving apparatus immediately upon the foundation-plate of the 

 stage by means of a strong dovetailed or conical ring, working firmly 

 but freely in the foundation-plate ; when in use it may be turned by 

 hand, or if great nicety be required, an endless screw may be fitted, 

 acting upon the ring, and connected with a milled head placed at the 

 side of the stage ; upon this ring is fixed a plate upon which the stage- 

 actions are adapted ; the adjustments are effected by two milled 

 heads on each side of the stage : the lateral motion consists of a 

 screw working in a collar connected with the sliding-plate, and the 

 vertical action is effected by means of a pinion fixed in the sliding- 

 plate and acting upon a rack on the under side of the top or object- 

 plate ; the whole is so constructed that it will traverse the entire 

 circle, and pass under the arm of the microscope without impedi- 

 ment. It may be objected as unnecessary that the stage and its ap- 

 purtenances should revolve entirely round the circle, and perhaps a 

 partial revolution would be sufficient for many objects, in which case 

 a larger and more commodious stage may be constructed, but my 

 principal object in proposing an entire revolution is to prove the effi- 

 cacy of the principle, as well as to adapt it in many cases where it 

 may be found advantageous, as, for example, in the examination 

 of opaque objects illuminated by a side light, when it is desirable to 

 cast a shadow in an opposite direction, which may be done by turning 

 the stage and object round instead of changing the light : it will also 

 be useful in examining certain objects in polarized light, when it 

 is wished to place either the neutral or positive axis of a crystal 

 to coincide with the plane of vibration. 



