154 



Laboratory Arts 



knitting-needle as before, when the marks are transferred 

 exactly as previously described in the case of the burette. 



The scale should then be numbered, and if it is to be 

 used for measuring upon, the zero mark should not be the first 

 but the second (see Fig. 114), as any fractional number may 

 then always be taken from the same part of the scale, and the 

 fractional division of the rest of the scale is unnecessary. Thus 

 to measure 3*4, one leg of the dividers would be placed upon 

 3, and they would be opened until the other reached - 0-4 ; 

 while in measuring 27 the fractional portion would be taken 

 from the same divided unit, consequently the possibility of 

 error would be greatly reduced. Should the scale be required 



FIG. 114. 



for lantern use, for light spot work, or needle indicator work, 

 however, it will need dividing throughout its whole length. 



When the transference of marks is completed, the waxed 

 brush may be applied to any marks which have been accident- 

 ally carried too far, and any other necessary obliterations made 

 in the same way. Name and date being added, a strip of 

 filter paper is cut as before, so as to cover the scale well, 

 soaked in a solution of hydrofluoric acid, and applied to the 

 scale. Any air-bubbles under the paper must be removed by 

 light pressure with a brush, or a plug of cotton-wool in the end 

 of a glass tube. The supply of hydrofluoric acid should be 

 liberal, but not so copious as to cause it to flow over the edges 

 of the filter paper. 



After ten minutes' treatment the paper is removed, the 

 scale well washed, heated, wiped clean and dry, polished with 

 benzene, and if perfect, rubbed with red lead, plaster of Paris, 



