354 PROCEEDINGS OF THE AMERICAN ACADEMY 



avoid possible change when once set. The eye-pieces are provided 

 with close-fitting collars, so that the draw tube may be removed aud 

 the eyepieces inserted directly into the body of the microscope. 



The micrometer stage was constructed by G. & S. Merz, of Munich. 

 It is their screw micrometer stage, adapted originally to their own 

 microscope stands. It consists essentially of a slide moving upon a 

 base plate 75 X 77 mm., and between two ledges adjusted with sides 

 parallel to the slide and the axis of the micrometer screw. The slide 

 in section is symmetrical, with its upper edge 3.35 cm., its lower edge 

 3.90 cm., and its vertical 5.0 mm. ; its upper surface has a length of 

 7.4 cm. The slide carries upon its upper surface another slide, which 

 by a rack aud pinion is moved at right angles to the axis of the screw. 

 This motion is necessary in order to assure an observer that lines of 

 the series he may be about to observe are placed at right angles to the 

 axis of the screw. 



The slide first mentioned is pulled by spiral springs with a force vary- 

 inrr from 0.7 kil. to 2.0 kil. niiainst the end of the screw, as the screw 

 moves the slide from one end of its run to the other, the bearing surface 

 being of steel. The nut through which the screw turns is fixed to the 

 lower plate on which the slide moves. This nut may be adjusted for 

 position, i.e. to render it concentric with the screw, and its friction on 

 the screw may be altered by turning a small screw which passes through 

 the nut on one side. This side has been cut through, so that the small 

 screw has really the nature of a clamp screw. 



The sliding plate carries a pointer indicating whole revolutions of 

 the screw on a silvered scale fixed to the lower plate. 



The screw itself is of steel, and it is cut as nearly as practicable 

 with 75 lines to the Paris inch. It is cut over a length of 26 mm., 

 and is 3.7 mm. in diameter at the bottom of the screw spiral. It has 

 the ordinary pattern micrometer head 46 mm. in diameter, which is 

 divided into 100 parts, each of which may be subdivided into 20 parts, 

 or even to a less degree by estimation by means of a mica scale and a 

 small magnifying lens. The nut is of red metal, and has an upper 

 surfiice rectangular in shape with a breadth of 14 mm. and i^ length 

 in the direction of the screw axis of 11.1 mm., thus preserving a ratio 

 of 3 : 1 with reference to the diameter of the screw. 



It might be remarked that this ratio is an old established one ; but 

 that Mr. Adam Hilger tells me he has lately constructed some small 

 screws, in which the relation of the nut to the diameter of the screw 

 was disregarded, but the nut was constructed f the length of the screw. 

 He spoke highly of his success with this construction. 



