Method of Using Abbes Apertometer. By E. M. Nelson. 593 
Now all these difficulties may be entirely overcome by simply 
discarding the use of the internal lens ; the procedure will then be as 
follows. Tlie apertometer is placed on the graduated rotating stage, 
and a low-power objective, say l|-in., is attached to the nose-piece ; 
the lamp with the edge of the flame towards the vertex of the aperto- 
meter is placed 12 or 15 in. in front of the Microscope, and is raised 
or lowered until the flame is level with the plane of the apertometer. 
The index of the graduated rotating stage is now placed to zero, and 
the hole in the silvered centring disc on the apertometer is brought 
into the centre of the field. The body of the Microscope is then 
racked up> until the apertometer is considerably beyond the focus. 
Now, if the image during this process is observed through the eye- 
piece, which should be of the lowest power, it will be seen that as the 
centring hole in the apertometer goes out of focus, a long stripe, 
which is an image of the edge of the flame, will come into focus. If 
this bright stripe is coincident with the out-of-focus image of the 
centring hole all is well ; but if it is not, the lamp must be moved 
either to the right or left hand until it becomes so. 
These are all the adjustments that are necessary, and they do not 
take a moment to arrange. The lens to be measured is then placed 
on the nose-piece, and the centring hole brought into focus, and re- 
centred to the field if it requires it, but the position of the lamp must 
not be disturbed. The next step is to rotate the stage until it reaches 
a point at which a dark shadow will be seen to gradually advance 
across the centring hole ; the stage must now be very slowly rotated, 
for when the shadow has reached a little more than half across the 
centring hole its motion will be greatly accelerated. When thi3 
accelerated motion takes place, the rotation of the stage must be 
stopped, and the angle through which it has been rotated read off. 
It may be as well to check the centring by reading off the angle when 
the stage has been rotated in the opposite direction. If the primary 
adjustments have been properly attended to, it will be found that the 
angle of rotation will be the same in both cases. A 1/4 in. is a 
good lens to experiment with in the first instance, and it will be found 
that, with a very little practice, the apertures of objectives can be 
accurately and quickly determined. 
The next step is to take out the sine of the angle thus found, and 
multiply it by the refractive index of the apertometer. There is a 
very handy little instrument called Boucher’s calculator, which saves 
all the labour of multiplication as well as the trouble of looking out 
the sine of the angle. The instrument is in the form of a keyless 
watch, and the procedure is as follows : — 
(1) Bring the refractive index of the apertometer on the outer 
circle of figures but one to the fixed index, by rotating the milled 
head at the bow. (2) Place the movable index to 1 on the scale, by 
turning the milled head at the side. (3) Bring the angle on the outer 
circle of figures to the movable index, by rotating the milled head at 
