158 THE MICROSCOPE. Dec. 
such as fine plumb and level lines, to many others besides 
myself. I first used them for the cross-wires of an ob- 
serving telescope about five years ago, while a student at 
Clark University. I have continued to use them ever 
since, whenever it was possible to obtain or make them, 
and I know of many others whose experience has been 
equally satisfactory. Nevertheless, they do not yet seem 
to be as generally known and widely used as they ought 
to be. It is sometimes difficult to obtain sufficiently fine 
fibres from dealers, at least in this country; but anyone 
can, after a little practice, make them for himself, the 
only really essential things being a good oxyhydrogen 
blow pipe and a supply of reasonably pure hydrogen (coal 
gas is very unsatisfactory) and oxygen in pressure tanks. 
