SOME SPECIAL PKOCESSES 261 



a wide dovetail-fitting upon which the stage racks up and 

 down. The limb is jointed on a centre so arranged that the 

 instrument is almost perfectly balanced when either vertical 

 or inclined at any angle. In the horizontal position it rests on 

 a cradle which forms part of the supporting-base, so that it is 

 then perfectly rigid.' The design is such that any tendency to 

 vibrate is much reduced ; in fact, any vibration transmitted 

 to the instrument would have the minimum effect in disturbing 

 any of its parts. The base has a very wide spread, so that in 

 this particular also great stability is obtained. The coarse- 

 adjustment has a range of 3| inches when the instrument is 

 vertical, and a further 1 inch when the instrument is at a slight 

 angle or horizontal. The fine-focussing is situated in the optio 

 axis immediately under the object. The stage is carried on 

 a triangular stem in a long triangular fitting in the stage- 

 support, and owing to the large surface of contact over 

 all these fitting parts, is also very rigid. The body-tube 

 is made of extra thickness ; and at the lower end is a nose- 

 piece for carrying the objective, with centreing-adjustments. 

 It follows that different objectives can be exactly adjusted to 

 the optic axis of the microscope, ensuring that on rotation of 

 the stage the object remains central. Three fittings are supplied 

 at the lower end. of ihe tube, into which all illuminating appli- 

 ances and their diaphragms may be attached to the instrument. 

 These include any type of vertical illuminator for throwing 

 direct light on to the object : the parabolic or Sorby illuminator, 

 already described, or a set of coloured-glass screens, suggested 

 by Mr. Eosenhain, for simultaneously illuminating with light 

 of different colours in different directions. A supplementary 

 draw- tube is provided for lengthening the body-tube when 

 this is required, and to allow of the use of very low-power 

 microscope objectives or photographic lenses. The stage 

 admits of complete rotation, and may be clamped in any required 

 position, allowing illumination to be effected at any angle. 

 The centre of rotation of the stage is as exactly as possible 

 at the optic axis of the instrument, and, as already mentioned, 

 centration is effected by adjusting the position of the objective 

 and not by altering the position of the stage itself. Mechanical 

 motion of the stage is provided in both directions. The 

 fittings for carrying the vertical illuminator are so arranged 



