POWELL AND LEALAND'S BEST STAND 195 



4-inch objective. The arm which carries the body is of unusual length 

 for the type it represents ; but this gives a large radius from the 

 optic centre of the instrument, aii<l makes the complete rotation of 

 the stage easy. Great efforts have been made to accomplish this in 

 other instruments. The older Ross form from the shortness of the 

 arm only allowed of a two-thirds rotation, and in the Lister model 

 many different devices have been tried, the latest being the placing 

 of the stage pinions in a vertical position above the stage, which is 

 an unquestionable error. 



The rotation of the stage in the Powell and Lealand model is by 

 means of a milled head most conveniently placed, and the divided 

 circle is on a plate of silver. 1 It will also rapidly rotate by hand. 



The arm is on a pivot, which allows it to be turned away from 

 the stage altogether, and, as we have already indicated, the length 

 of the arm lent itself to the use of, a longer lever for the fine adjust- 

 ment (p. 174). The milled head is" placed behind the strong pivot 

 of the arm, where vibration is impossible, and it is in an easy and 

 natural position for the access of either hand. 



The body may be, with great ease, entirely removed from the arm ; 

 this makes the use of the binocular or monocular body or of a short 

 or long body a matter of choice, while it gives access for cleaning and 

 other purposes to the nose-piece tube, as well as for the insertion 

 and focussing of the lens used with an apertometer, 2 or an analysing 

 prism. So also it is of service in low-power photo-micrography. 



We have already referred to the stage of this instrument ; 

 but it may be briefly stated that it is large, has complete rotation, 

 it has one inch of rectangular motion, being graduated to the r tn>th 

 inch for a finder. There is the same speed in the vertical and the 

 lateral movements, and the pinions do not alter their positions. The 

 aperture of the stage is amply large. 



The ledge of the stage has a stop placed on its left-hand 

 side ; this is held by a screw, but is removable at pleasure. 

 Two massive brackets under the stage remove all possibility of 

 flexure. 



The sub-stage has rectangular movements by screw in either direc- 

 tion, as well as a rotary movement by pinion. The coarse adjust- 

 ment is by rackwork, and a fine adjustment is added when desired. 

 Fig. 158 illustrates this stage, showing its under side in order to 

 enable the fine adjustment to be seen. 



The vertical and upper horizontal milled heads are centring 

 screws, acting at right angles to each other, while the diagonal screw 

 to the left is the milled head, which causes the stage to rotate, the 

 whole acting with great smoothness and accuracy, also enabling the 

 operator to centre with complete precision, while, as we have 

 already seen (pp. 187 and 196), the milled head A works by an 

 advancing cone the fine adjustment to this stage. 



The mirror is plane and concave, with double-jointed arm. 



The finish and workmanship of this instrument are of the highest 

 order. The seen and the unseen receive equally scrupulous care. 



1 This is now made of platinum if desired, and thus tarnish is obviated. 



2 Chapter V. p. 337. 



02 



