Angle of Aperture of Object-glasses. By F. II. Wenham. 213 
direction of tlie eye in the axis, and prevents any rays except those 
of the central pencil from entering. By means of the draw-tube a 
lamp-flame or other object, taken for an index, is focussed for 
distinct vision without the stop. Beplace this and take the angle of 
the objective, either by rotating on a sector, in the usual way, or by 
measuring the angle between two objects set the requisite distances 
asunder, the apex being at the focal point of the object-glass. A 
stop as small as one-fiftieth of an inch may be used without incon- 
venience, as the speck of light from the flame can be plainly seen, 
and the error in excess of true aperture will not be worth allowing 
for Of course a large stop will give inaccurate results, by admitting 
oblique pencils into the eye. 
In order to show the distinction between angle of aperture and 
angle of field, I quote the following experiment, which also illus- 
trates the fallacy of the usual mode of measurement. A 11- inch 
object-glass was taken, consisting of two superposed achromatics. 
The aperture by the usual method was 12°. The exact distance of 
the focus from the front face of lens was one inch and three- 
tenths. A thin metal stop was then fitted close on to the front lens, 
having a central aperture of ■ 046 in. Now from this dimension to 
the focus the pencil of rays must be absolutely limited to an angle 
of two degrees ; yet when measured by the usual sector method, with 
this stop in place, the angle of aperture indicated was more than 
double this, or five degrees. The difference in excess belonged to 
angle of field instead of angle of aperture. 
Some object-glasses, said to have an aperture of near 180°, are 
reduced by correct measurement to near one-half’: an axial pencil 
having an angle of 100 J is rarely to be met with. If a finely 
embossed or corrugated plane is viewed at various degrees of incli- 
nation, there is a certain angle of sight that gives the maximum of 
distinctness. It is about 45° or 50°. Beyond this, as the eye 
approaches nearer to the level of the plane, the vision becomes 
more confused and indistinct, till, at a few degrees, the structure is 
quite obscured. This observation only applies to the degrees of 
vision. For illumination, light may be thrown on the object at the 
utmost obliquity of incidence with advantage, in order to obtain the 
contrast between light and shadow, in prominences and depressions. 
