This may be done by examining the spaces included between successive parallels and meridians, near the edges and angles of the maps, and comparing these either with those in the centre of the map, or with the known figures and dimensions of the corresponding spaces on a globe.
We may now proceed to discuss the different tests which the intending purchaser of a telescope should apply to the instrument.
The excellence of an object-glass can be satisfactorily determined only by testing the performance of the telescope in the manner presently to be described.

But it is well to examine the quality of the glass as respects transparency and uniformity of texture.

Bubbles, scratches, and other such defects, are not very important, since they do not affect the distinctness of the field as they would in a Galilean Telescope,--a little light is lost, and that is all.

The same remark applies to dust upon the glass.

The glass should be kept as free as possible from dirt, damp, or dust, but it is not advisable to remove every speck which, despite such precaution, may accidentally fall upon the object-glass.
When it becomes necessary to clean the glass, it is to be noted that the substance used should be soft, perfectly dry, and free from dust.