264 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



position/ it is not provided with a jointed pillar to secure inclination. 

 The coarse adjustment is by rack and pinion ; the fine, by the usual 

 micrometer screw of this firm. The stage is circular and rotates, 

 being provided with centring screws, and its margin is graduated 

 into degrees for measuring crystal angles. Except for this graduated 

 circle the stage is faced with hard rubber. The sub-stage is adjust- 

 able by means of a quick-acting screw. This is fitted with polarising 

 apparatus, consisting of a large Nicol prism so mounted that by 

 means of a pin fitting into a slot in the sub-stage the prism can 

 always be replaced in exactly the same position, and rotated with a 

 circle graduated in degrees ; or it can be swung aside when polarised 

 light is not needed. The analysing Nicol prism is also provided 

 with a graduated circle, and is so mounted that it fits over and above 

 any eye-piece. The draw-tube of the microscope is furnished with 

 a small projecting pin, which fits into a slot cut in the bottom of 

 the tube-mounting of the analyser. This slot lies in the same 

 vertical plane as the zero points of the analyser, the polariser, and 

 the stage. The zero points of the two former are arranged as usual 

 for the position of crossed Nicols ; hence, when the polariser is in 

 position and at zero, and the analyser is at zero and is in position 

 by its pin and slot, the Nicols are crossed without further adjust- 

 ment ; this, of course, saves much time. But it is clearly a simplified 

 petrological microscope ; it is not intended for petrological or 

 mineralogical work, it is simply an instrument made at a very 

 low price, but stated by Prof. Chamot to be competent for all 

 chemical work or food examinations. 



An equally important special form of microscope has been made 

 by Reichert for the examination of metals. 1 Fig. 207 shows this 

 instrument made according to the instructions of Dr. A. Rejto, of 

 Budapest. In general appearance it resembles the ordinary horse- 

 shoe stand, but it has no mirror, and the stage, which is made 

 adjustable in height, may also be removed altogether. 



With very low powers the specimen may be illuminated by 

 diffused daylight or artificial light falling freely upon its surface. 

 With higher powers an illuminator is used which fits the tube of 

 the microscope, and is provided with an extension to receive the 

 eye-piece. The illuminator consists of a thin plate of glass placed 

 at an angle of 45 with regard to the axis of the tube, and of a con- 

 densing lens whose focal length is equal to the sum of distances 

 between the lens and the plate of glass, and between the latter and 

 the object. 



The question of illumination is a very important one, to whic 

 great attention is to be devoted. 



As source of light the ' Auer,' a triplex burner, adjustable in 

 height, may be recommended ; 2 it is placed at a distance of one 

 metre from the illuminator. The flame is surrounded by an iron 

 or asbestos cylinder, with only the necessary aperture for illumination 

 of the object. The source of light should be at exactly the same 

 level with the lens, 6, of the illuminator. On removing the eye- 



1 Central-ZeMung fur Optik und Mechanik, No. 17, 1897. 



2 Supplied by Reichert. 



