Messrs. Walton and Maberly in an octavo work; and my own 'Constellation-Seasons' give, at the same price, twelve quarto maps (of four of which those in Plate I. are miniatures), showing the appearance of the sky at any hour from month to month, or on any night, at successive intervals of two hours. But maps intermediate in character to these and to Observatory maps are required by the amateur observer. Such are the Society's six gnomonic maps, the set of six gnomonic maps in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic maps. The Society's maps are a remarkably good set, containing on the scale of a ten-inch globe all the stars in the Catalogue of the Astronomical Society (down to the fifth magnitude). The distortion, however, is necessarily enormous when the celestial sphere is presented in only six gnomonic maps. In my maps all the stars of the British Association Catalogue down to the fifth magnitude are included on the scale of a six-inch globe. The distortion is scarcely a fourth of that in the Society's maps. The maps are so arranged that the relative positions of all the stars in each hemisphere can be readily gathered from a single view; and black duplicate-maps serve to show the appearance of the constellations.

It is often convenient to make small maps of a part of the heavens we may wish to study closely. My 'Handbook of the Stars' has been prepared to aid the student in the construction of such maps.

In selecting maps it is well to be able to recognise the amount of distortion and scale-variation. 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. Silk is often recommended, but some silk is exceedingly objectionable in texture,—old silk, perfectly soft to the touch, is perhaps as good as anything. If the dust which has fallen on the glass is at all gritty, the glass will suffer by the method of cleaning commonly adopted, in which the dust is gathered up by pressure. The proper method is to clean a small space near the edge of the glass, and to sweep from that space as centre. In this way the dust is pushed before the silk or wash-leather, and does not cut the glass. It is well always to suspect the presence of gritty dust, and adopt this cautious method of cleaning.

The two glasses should on no account be separated.

In examining an eye-piece, the quality of the glass should be noted, and care taken that both glasses (but especially the field-glass) are free from the least speck, scratch, or blemish of any kind, for these defects will be exhibited in a magnified state in the field of view. Hence the eye-pieces require to be as carefully preserved from damp and dust as the object-glass, and to be more frequently cleaned.

The tube of the telescope should be light, but strong, and free from vibration. Its quality in the last respect can be tested by lightly striking it when mounted; the sound given out should be dead or non-resonant. The inside of the tube must absorb extraneous light, and should therefore be coloured a dull black; and stops of varying radius should be placed along its length with the same object. Sliding tubes, rack-work, etc., should work closely, yet easily.

The telescope should be well balanced for vision with the small astronomical eye-pieces. But as there is often occasion to use appliances which disturb the balance, it is well to have the means of at once restoring equilibrium. A cord ring running round the tube (pretty tightly, so as to rest still when the tube is inclined), and bearing a small weight, will be all that is required for this purpose; it must be slipped along the tube until the tube is found to be perfectly balanced. Nothing is more annoying than, after getting a star well in the field, to see the tube shift its position through defective balance, and thus to have to search again for the star. Even with such an arrangement as is shown in fig. 8, though the tube cannot readily shift its position, it is better to have it well balanced.

The quality of the stand has a very important influence on the performance of a telescope. In fact, a moderately good telescope, mounted on a steady stand, working easily and conveniently, will not only enable the observer to pass his time much more pleasantly, but will absolutely exhibit more difficult objects than a finer instrument on a rickety, ill-arranged stand. A good observing-chair is also a matter of some importance, the least constraint or awkwardness of position detracting considerably from the power of distinct vision. Such, at least, is my own experience.

But the mere examination of the glasses, tube, mounting, &c., is only the first step in the series of tests which should be applied to a telescope, since the excellence of the instrument depends, not on its size, the beauty of its mounting, or any extraneous circumstances, but on its performance.

The observer should first determine whether the chromatic aberration is corrected. To ascertain this the telescope should be directed to the moon, or (better) to Jupiter, and accurately focussed for distinct vision. If, then, on moving the eye-piece towards the object-glass, a ring of purple appears round the margin of the object, and on moving the eye-glass in the contrary direction a ring of green, the chromatic aberration is corrected, since these are the colours of the secondary spectrum.

To determine whether the spherical aberration is corrected, the telescope should be directed towards a star of the third or fourth magnitude, and focussed for distinct vision. A cap with an aperture of about one-half its diameter should then be placed over the object-glass. If no new adjustment is required for distinct vision, the spherical aberration is corrected, since the mean focal length and the focal length of the central rays are equal. If, when the cap is on, the eye-piece has to be pulled out for distinct vision, the spherical aberration has not been fully corrected; if the eye-piece has to be pushed in, the aberration has been over-corrected. 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