XIV 



Proceedin(]S. [Dcca/iher i6ll/, igo2. 



a piece of tube of fairly wide bore with its axis vertical, so 

 that its diameter may be measured directly. 



The arrangement for reading is similar to the cursor used 

 with a slide-rule. A thin plate of transparent celluloid is fixed 

 in a metal frame which can slide freely and steadily over the 

 scales and groove {Fig. j). On the under surface of the celluloid is 



/•>>. J. Skelch of slider, lens, &c. 



marked a fine transverse straight line, which is adjusted, by 

 sliding the metal frame, so as to be exactly over the end of the 

 mercury thread to be measured. A short strip of mirror glass 

 is fixed to the under surface of the celluloid, so that parallax 

 errors may be avoided. The readings on the scales are taken 

 by the aid of two verniers attached to, or engraved on, the 

 under surface of the celluloid plate, each vernier having ten 

 divisions corresponding to nine divisions on the main scales. 



The observations are made through a simple microscope, 

 or reading lens, carried by the sliding frame. The lens is 

 supported by a vertical pillar so that its centre is exactly over 

 the cross line on the celluloid plate, and at a suitable height 



