118 MECHANICAL ARRANGEMENT OF THE MICROSCOPE. 



powers are used, is generally due to the object-slide. Where the 

 certainty of the observation may be impaired by this reflected 

 light, it must be excluded either by the hand or other suitable 

 means. 



The aperture of the stage should be at least as large as the 

 field of view of the lowest-power objective. As, however, for 

 many researches a much smaller aperture is desirable, and for 

 others a much larger one, of at least 1 inch in diameter, the stage 

 itself should have a large aperture which may be reduced to the 

 desired size by means of perforated discs, which drop into the 

 central hole to a suitable depth. This arrangement is convenient 

 for other purposes for instance, the application of selenite plates 

 in investigations with polarized light. 



Slide-grooves attached beneath the stage are advantageous ;, 

 the whole diaphragm-apparatus may be thus moved laterally at 

 pleasure, or removed altogether, which is important in many 

 physical researches. 



Large stands should be constructed so that the stage and the 

 whole upper part of the Microscope may be revolved together on 

 the optic axis ; the incident light then strikes the object at any 

 desired angle in azimuth. This arrangement, for which we are 

 indebted to Oberhseuser, is now generally adopted in large 

 Microscopes. In the smaller stands, this system of rotation may 

 be replaced by a revolving stage-plate, which should be provided 

 with centering arrangements. For since the slightest excentricity of 

 the axis of revolution produces marked alterations of position of the 

 object during the turning which readily increase till the object 

 vanishes from the field of view, and on the other hand the optic 

 axis of the different objectives never lie exactly in the same 

 vertical it is impossible to adjust the plate accurately once for 

 all ; it must therefore admit of being moved at right angles within 

 certain limits. In micrometric and angular measurements the 

 centering movements facilitate the exact adjustment of division- 

 lines or crossed threads on a particular point, and are thus of 

 great service in crystallographical investigations. This rotating 

 stage-plate should be removable. If it is graduated on the 

 circumference, as in the large instruments of Seibert and Krafft, 

 it will be serviceable for the measurement of angles. 



The Illuminating Apparatus. In accordance with our discus- 

 sion of the principles of illumination, the apparatus may differ 



