Mercury Globules as Test Objects. By J. W. Gordon. 19 



mercury globules it is possible to obtain test objects of all dimen- 

 sions down to such as will be invisible in the finest instruments 

 that can be made, while at the same time we have the power to 

 vary the size of our test object at any point in the series by the 

 finest degrees of change of magnitude. 



It will do doubt be understood that in the case of the speculum 

 globule it is necessary, and in the case of the object globule con- 

 venient, to have it mounted between glasses in Canada balsam. In 

 the case of the speculum globule, which reflects the image from its 

 lower face, it is necessary to make sure that the lower face is not 

 resting in contact with the glass. If the balsam is at all viscous, 

 the globule may subside into that position in use, and so present a 

 flattened face to the object globule which, of course, entirely alters 

 the dimensions of the resulting second order image. 



EXPLANATION OF PLATE I. 



Fig 1.— Mercury Globule. 

 „ 2.— Etched Tinfoil. 



