8 
THE YOUNG SCIENTIST. 
Another consideration now presents itself, 
for we find that the position in which the work- 
man stands with regard to the stone has an 
important bearing upon the amount of feather 
edge that will be formed. Suppose, for in- 
stance, that Fig. 4 represents a section of 
a grindstone running in the direction of the 
arrow B, and that C and D represent sections 
of two knife blades; then C would have a long 
feather edge formed upon it, while D would 
Fig. 6. 
moderately sharp edge; but at whatever angle 
the blade is first held to the face of the oil- 
stone, it should be maintained there, otherwise 
the action will be to round off the edge. The 
motion necessary during the oilstonicg is 
shown in Fig. 7, the blades lying diagonally 
across the stone. The motion of one side of 
the blade being from A to B, the position at A 
being shown in full lines, and at B in dotted 
lines; and of the other side from C to D, at C 
Fig. 7. 
not, and the latter is the best position; but if 
the stone has any very soft place in it, the 
blade held in that position is apt to catch, and 
if the stone is out of true, it is sure to do so 
more or less, and as it cannot then be held 
steadily, the face ground will be wavy, and a 
good edge becomes impossible. 
To obtain a finer cutting edge, we must have 
recourse to a finer abrasive, or, in other words, 
to an oilstone. The action of the 
J oilstone is to smooth the surfaces, 
and thus remove the feather edge, 
but while doing this the oilstone 
itself turns the edge over, forming 
what is known as a wire edge, which 
is shown in Fig. 5 at A, which is 
Fig. 5. formed from the causes already ex- 
plained in referring to the formation 
of feather edges, and which resembles the 
feather edge, except that it is smooth, and to a 
greater extent continuous. In holding the 
blade to the oilstone, it is necessary to keep 
the face of the blade level, as at B, Fig. 6, or as 
nearly so as possible with the oilstone, while at 
the same time the pressure of the blade is 
greatest towards the cutting edge. 
But if the blade has been frequently sharp- 
ened upon the oilstone since it was last ground 
upon a grinding stone, it may be slightly ele- 
vated, (as shown considerably exaggerated in 
Fig. 6 at A), which will give a durable and a 
in full, and at D in dotted lines— the blade 
being frequently turned over, and the strokes 
being short and made under light pressure, so 
as to make the wire edge as small as possible. 
Penny Microscopes. 
Many years ago a man in London made a living 
by selling " Penny Microscopes," which would ex- 
hibit the eels in paste, the scales on the butterfly's 
wing, and a few other common objects. Dickens, 
in his "Household Words," has given a most 
graphic description of the microscopes, the man 
and his family, every member of which performed 
-:nme part toward the production of the little in- 
strument. 
i These microscopes were made out of small boxes 
I resembling pill boxes, in the lid of which was fixed 
the lens, while the bottom served to support the 
object, a hole having been made to allow the light 
to pass through, so that eels and similar objects 
might be viewed by holding the box up against the 
light. The distance between the lid and the bot- 
tom was very nearly equal to the focal length of 
the lens, and might be adjusted by slipping the 
lid on or off. The interior of the box was of course 
blackened. 
The lens or magnifying power was, of course, a 
very simple affair. In this case it was nothing 
more than a drop of balsam of fir placed in a 
smoothly cut hole about an eighth of an inch in 
diameter. And yet such a microscope, if skilfully 
made, would do quite as good work as the brass 
affairs which are now so commonly sold in the 
