As a further test, we may cut off the central rays, by means of a circular card covering the middle of the object-glass, and compare the focal length for distinct vision with the focal length when the cap is applied.

The extent of the spherical aberration may be thus determined; but if the first experiment gives a satisfactory result, no other is required.
A star of the first magnitude should next be brought into the field of view.

If an irradiation from one side is perceived, part of the object-glass has not the same refractive power as the rest; and the part which is defective can be determined by applying in different positions a cap which hides half the object-glass.

If the irradiation is double, it will probably be found that the object-glass has been too tightly screwed, and the defect will disappear when the glass is freed from such undue pressure.
If the object-glass is not quite at right angles to the axis of the tube, or if the eye-tube is at all inclined, a like irradiation will appear when a bright star is in the field.

The former defect is not easily detected or remedied; nor is it commonly met with in the work of a careful optician.