592 
Transactions of the Society. 
XI. — A Simplification of the Method of Using Professor Abbes 
Apertometer. 
By E. M. Nelson, F.R.M.S. 
( Read 21s t October , 1896.) 
Wben Professor Abbe’s apertometer was first brought out, a difficulty 
was experienced in observing the precise point at which images of 
the sliding screens indicated the limit of the aperture of the objective. 
To overcome this difficulty another way of using the instrument 
was devised, which dispensed with the use of the screens altogether. 
This is effected by placing the semicircular disc on a Microscope, the 
stage of which is fitted with a graduated rotary movement. When 
the lens, the aperture of which is to be measured, has been focussed, 
and the semicircular disc centred to its axis, the interior lens also 
having been attached and adjusted, the image of the edge of the flame 
of an ordinary Microscope lamp is caused to travel across the 
aperture of the objective by the rotation of the stage until the image 
of the flame is extinguished at the limit of the aperture. This 
extinction of the image of the edge of the flame affords a nice point 
by which to determine the limit of the aperture ; for this reason this 
improved method at once became general among microscopists in this 
country. 
Now, if the adjustments and centring of the lense3 and the lamp 
have been properly attended to, it will be found that the stage 
must be rotated through equal angles on either side of zero for the 
extinction of the image of the elge of the flame. Then, in order to 
determine the N.A. of the objective, all that is needed is to multiply 
the sine of the angle thus found by the refractive index marked on 
the semicircular glass plate, the product being the required N.A. ; of 
course, if the angles on either side of zero are not the same, the 
sine of their mean value is taken. 
The objections to this method are two in number. The first is, 
that *a Microscope with a graduated rotating stage is necessary. In 
reply to this, we may say that the ordinary student who uses an 
elementary Continental form of stand without a graduated rotating 
stage, is not likely either to possess or use an apertometer. Those 
microscopists who have and use apertometers in all probability possess 
also a Microscope with a graduated rotary stage. 
The second objection, a more serious one, is that unless the Micro- 
scope is of the Powell type with a removable body, it is often a very 
difficult, and sometimes an impossible matter, to adapt and adjust the 
internal lens. Even when the body is removable, most microscopists 
have experienced, for various reasons which it is unnecessary to enter 
into here, some trouble in the adjustment of this lens. 
