A few notes may be added with regard to observation. In drawing the planet regard must first of all be paid to the fact that Jupiter's disc is not circular, and should never be so represented. It is easy for the student to prepare for himself a disc of convenient size, say about 2-1/2 inches in diameter on the major axis, and compressed to the proper extent (1/16), which may be used in outlining all subsequent drawings. Within the outline thus sketched the details must be drawn with as great rapidity as is consistent with accuracy. The reason for rapidity will soon become obvious. Jupiter's period of rotation is so short that the aspect of his disc will be found to change materially even in half an hour. Indeed, twenty minutes is perhaps as long as the observer should allow himself for any individual drawing, and a little practice will convince him that it is quite possible to represent a good deal of detail in that time, and that, even with rapid work, the placing of the various markings may be made pretty accurate. The darker and more conspicuous features should be laid down first of all, and the more delicate details thereafter filled in, care being taken to secure first those near the preceding margin of the planet before they are carried out of view by rotation. The colours of the various features should be carefully noted at the sides of the original drawings, and for this work twilight observations are to be preferred. Different observers vary to some extent, as might be expected, in their estimates of the planet's colouring, but on the whole there is a broad general agreement. No planet presents such a fine opportunity for colour-study as Jupiter, and on occasions of good seeing the richness of the tones is perfectly astonishing. In showing the natural colours of the planet the reflector has a great advantage over the refractor, and observers using the reflecting type of instrument should devote particular attention to this branch of the subject, as there is no doubt that the colour of the various features is liable to considerable, perhaps seasonal, variation, and systematic observation of its changes may prove helpful in solving the mystery of Jupiter's condition. The times of beginning and ending observation should be carefully noted, and also the magnifying powers employed. These should not be too high. Jupiter does not need, and will not stand, so much enlargement as either Mars or Saturn. It is quite easy to secure a very large disc, but over-magnifying is a great deal worse than useless: it is a fertile source of mistakes and illusions. If the student be content to make reasonable use of his means, and not to overpress either his instrument or his imagination, he will find upon Jupiter work full of absorbing interest, and may be able to make his own contribution to the serious study of the great planet. CHAPTER XI SATURN