486 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



from a mirror set in a universal joint in the base-plate. The drawing- 

 board is pushed up and down on a desk-shaped staud at an angle of 25°. 

 This frame can be slid backwards and forwards in a groove of the 

 base-plate. Scales are set on both sides of the object-table, and, in con- 

 nection with the scale on the base-plate, have the effect of preserving 

 the same magnification in a drawing of any part of a large preparation. 

 It is also possible to draw in reduced measure, if the preparation is set 

 on the drawing-board and the paper on the object-table. 



Photo-measuring Micrometer. — A. Hilger's photo - measuring 

 micrometer (fig. 98), though specially designed for accurate and rapid 



Fig. 98. 



measurements of spectrum photographs, is adapted for general labora- 

 tory work. The accuracy of the instrument is attained by the careful 

 manufacture of the steel screw, its nut, and the gun-metal Microscope- 

 slide. Its durability is ensured by simplicity of design, by the pro- 

 vision of adequate surface-bearings on the screw and on the Microscope- 

 slide, and by the deep thread of the screw and its substantial diameter 

 of ^ in. As the pitch of the screw is 1 mm., and there is a large divided 

 drum-head, the whole traverse of 6 in. can be rapidly worked over and 

 exceedingly accurate measurements taken. The base of the instrument 

 is of cast iron, a'nd the Microscope-slide is mounted on two cast iron 

 standaids. 



