On photographs which have been taken with long exposures the stellar images are large, so that a considerable amount of uncertainty is involved in bisecting them with accuracy when measuring position angles of and distances between such stars; and the method of making several measurements and resetting the instrument for each one in order to obtain a mean position for the centre is troublesome. But if with a fine needle point the centre of the photographic disc could be accurately punctured, one careful measurement made from such a well-defined centre point would be more reliable than the mean of several approximate measurements.

The following method of puncturing the star discs I have found in practice to give satisfactory results:–

A microscope is taken that has a revolving stage and also a sub-stage for holding and centering an achromatic condenser. By removing the front lens of the condenser and replacing it with a brass cap in which is inserted a fine needle point at its centre, we obtain a fixed point which can by the screw adjustments of the condenser be accurately placed in the optical axis of the microscope. In the ocular is placed, through the ordinary micrometer slit, a finely ruled glass réseau or a spider-line cross with the intersection of the cross, or of any two lines of the réseau at right angles brought into the optical centre of the instrument.