492 SUMMARY OF CURRENT RESEARCHES RELATING TO 



bar supported on two feet, and graduated into millimetres; at the back 

 of the base plate there is a sill plate planed on top and parallel to the 

 prism bar. The plate-stage (or object-stage) is carried on two bearers 

 moving on the prism bars and supported by rollers, the bearers being 

 actuated by rack-and-pinion. The base-plate also carries an arched 

 support at right angles to the stage movement ; the summit of this 

 arch is another prism bar, and carries the Microscope on bearers actuated 

 by rack-and-pinion. The Microscope movement is naturally at right 

 angles to the stage movement. The upper prism-bar is graduated into 

 millimetres, but both prisms can be more finely graduated if desired. 

 A position circle on the stage is intended to receive photographic plates 

 up to 16 by 16 cm., and is connected with a circular rackwork under 

 this stage controlled from the right-hand end of the stage. A frame, 

 clearly shown in the illustration, covers the object placed on the posi- 

 tion-circle, and contains a grating divided into intervals of 2 by 2 mm. 

 This frame moves on a hinge (seen to the left), and is kept tight, when 

 shut up, by a screw. The Microscope magnifies ten times, and can be 

 rotated in a long groove 90° about its optic axis ; it can be clamped 

 firmly on an adjustable peg, so that the micrometer screw of the Micro- 

 scope is parallel to one or another of the lines of the grating-system. 

 The Microscope measurement is, therefore, merely applied from line to 

 line of the glass plate (at most 2-2 lines). The divisions on both 

 prisms correspond to the glass net, and should he parallel with them; 

 therefore, the divisions on the prisms should coincide with the net- 

 lines, and this is easily regulated by the index. The index on the prism 

 graduations, as well as on the position circle, is easily read by means of 

 large loups of convenient size. This apparatus has been in use for four 

 years at the Prussian Royal Astrophysical Observatory, and has given 

 satisfactory results. 



Vogel-Hale Measuring Microscope (Model C).* — This instrument 

 is listed No. &c in the maker's catalogue, and is shown in fig. 123. It is 

 mainly intended for the measurement of solar spectra. The strong iron 

 stand on which it is mounted can lie inclined at any angle between 0° 

 and 60° at the observer's pleasure. The iron frame forming the 

 measuring stage slides between two steel runners, and is covered with a 

 glass plate for the reception of the object, which is secured by pressure 

 springs of adjustable length. The measuring screw is very carefully 

 constructed, and has an available length of 150 mm. ; one rotation of 

 the thread gives an axial movement of 0*5 mm., and imparts a corre- 

 sponding movement to the measuring stage by means of a steel nut 

 beneath it. A counterweight is applied to the screw so as to avoid 

 deadway. Two drums, with common index, are fitted near the screw- 

 head, and give the readings : one of these drums records the rotations of 

 the screw, and the other the rotations of the first drum. The first drum 

 is divided into hundredths, and tenths of these can safely be estimated, 

 so that a reading of 0*0005 mm. can be obtained ; a scale divided into 

 millimetres shows the movement of the stage in that unit. The illu- 



* Otto Toepfer und Sohn's Catalogue (Neue Astrophisikalische Apparate, 

 1908), Potsdam. 



