Angle of A]gerture of Ohjectives. By F. II. Wenham. 325 
the degrees of the entire lens, and twice the difference is the 
aijerture. 
By this simple rule we eliminate the angle of field, which has 
hitherto been erroneously added to angle of aperture. In order to 
obscure the half of the front lens, take a small piece of tinfoil with 
a clean-cut edge, lay this on a glass slip, and smear its upper 
surface with a bit of dry soap (anything moist will bedew the 
glass), place this on the microscope stage, and bring the edge in 
focus under the object-glass, until it exactly bisects the field 
vertically. Then lower the object-glass on to the foil, which will 
adhere to the front lens. The microscope body may then he laid 
horizontally, and the apparent aperture of the open half of the lens 
measured by the sector, which invariably indicates considerably 
more than half of the full aperture, taken by the same method. 
In ohjectives of large angle, twice this half measurement will 
amount to absurd and impossible angles. 
The application of the above rule will require to be thoroughly 
investigated before it can be finally adopted. In every case it 
brings out a reduction on the degrees of aperture indicated by all 
the modes of measurement in present use. I give its test, on what 
I have repeatedly stated to be fabulous apertures, viz. 180 b I 
liave one of these ohjectives— a yet in ridiculous disproof of 
such an aperture (which would bring the focal plane on to the surface 
of the front lens), it is remarkable for its working distance, and will 
penetrate through the cover of any object in my cabinet, and used 
with thick covers, either as immersion or dry, its performance is 
very fine. Taken by the sector, the open aperture, or what I will 
term angle of field, is 180T The aperture of half this lens is 115°, 
twice the difterence is 130'; this represents the degrees of the 
central pencil, which is the true aperture. I take another example 
of an immersion lens, in which the maker actually claims “plus 
180°.” Oil trying this by the sector, the light image vanished 
at a range of 178°, beyond which it was totally obscured. The 
aperture of half the lens was 114°, twice the difference is 128°. 
