ZOOLOGY AND BOTANY, -MICROSCOPY, ETC. 



499 



removed so as to expose the threads. The Microscope of the measuring 

 apparatus is sharply directed on the threads, whose orientation is judged 

 by the position angle of the stage. Illumination is by a mirror. The 

 screw which operates the Microscope is an accurately worked micrometer 

 screw, and thus also serves for measurement. Its thread-distance is 

 1 mm., its available range 80 mm., the whole rotations being read off 

 on a straight-edged scale, and the hundredths on a drum at the lower 



Fig. 12S. 



end of the screw ; the accuracy is to TT5 Vo mm. A disk 0, with finger 

 openings, is provided for quick motions. The dead weight of the 

 Microscope is taken off the bearings as far as possible by a suspended 

 weight p, so that the sliding movement is extremely smooth. The 

 Microscope has three objectives, and is focused by rack-and-pinion ; 

 the magnifying powers are known by reference to a graduated scale on 

 the draw-tube. The upper end of the Microscope is defined by a circular 

 flange, and has two independent rotations, one of which may extend to 



2 L 2 



