OF THE MICROSCOPE. Ill 



In some forms the glass plate is stationary, and the object- 

 carrier moves over it, the points of contact being very small. 



Revolving Stage. It is often desirable to rotate an 

 object in .the optic axis of the microscope, either for the pur- 

 pose of measuring angles or changing the direction of the 

 illumination in regard to the object. Means for effecting this 

 with perfect accuracy have been applied both to the mechanical 

 stage and the glass stage, though the latter is generally so con- 

 structed that rotation is impossible. In the large Koss stand (PI. 

 I), the stage rotates, and is graduated for measuring angles. 



M. Xachet has devised a special form of the glass stage, in 

 which provision is made for rotating it. In this form of the 



stage the object-earner, F, is 

 held to the glass plate, E, by 

 means of two springs with ivory 

 points, the springs being at- 

 tached to the frame in which 

 the glass plate, E, is set. Both 

 the plate, E, and the object- 

 carrier rotate in a light brass 

 frame. In all these cases an 

 attempt is made to secure coin- 

 cidence of the centre of rota- 

 tion with the optic axis of the 



instrument. It will, however, be found that it is an impossi- 

 bility to attain this object with any great degree of accuracy 

 unless provision is made for occasionally adjusting the centre 

 of rotation of the stage. No stage that ever was made will re- 

 main for any length of time so true that angles can be correctly 

 measured by it, and therefore several devices have been sug- 

 gested for securing accuracy without adjusting the stage, since 

 when Avell-made an adjustable stage is expensive, and when 

 poorly made it is worthless. 



The best known of these devices is what is called the center- 

 ing nose-piece. This is a kind of adapter which is screwed on 

 the body and receives the objective. It is provided with center- 

 ing arrangements, and the objective having been screwed into 



