Plates [III.] and [V.] represent fifty-two of the objects mentioned in the above-named chapters. As reference is made to these figures in the text, little comment is here required. It is to be remarked, however, that the circles, and especially the small circles, do not represent the whole of the telescope's field of view, only a small portion of it. The object of these figures is to enable the observer to know what to expect when he turns his telescope towards a difficult double star. Many of the objects depicted are very easy doubles: these are given as objects of reference. The observer having seen the correspondence between an easy double and its picture, as respects the relation between the line joining the components and the apparent path of the double across the telescope's field of view, will know how to interpret the picture of a difficult double in this respect. And as all the small figures are drawn to one scale, he will also know how far apart he may expect to find the components of a difficult double. Thus he will have an exact conception of the sort of duplicity he is to look for, and this is—crede experto—a great step towards the detection of the star's duplicity.
PLATES [VI.] and [VII.], illustrating Chapters [VI.] and [VII.]
The views of Mercury, Venus, and Mars in these plates (except the smaller view of Jupiter in Plate [VII.]) are supposed to be seen with the same "power."
The observer must not expect to see the details presented in the views of Mars with anything like the distinctness I have here given to them. If he place the plate at a distance of six or seven yards he will see the views more nearly as Mars is likely to appear in a good three-inch aperture.
The chart of Mars is a reduction of one I have constructed from views by Mr. Dawes. I believe that nearly all the features included in the chart are permanent, though not always visible. I take this opportunity of noting that the eighteen orthographic pictures of Mars presented with my shilling chart are to be looked on rather as maps than as representing telescopic views. They illustrate usefully the varying presentation of Mars towards the earth. The observer can obtain other such illustrations for himself by filling in outlines, traced from those given at the foot of Plate [VI.], with details from the chart. It is to be noted that Mars varies in presentation, not only as respects the greater or less opening out of his equator towards the north or south, but as respects the apparent slope of his polar axis to the right or left. The four projections as shown, or inverted, or seen from the back of the plate (held up to the light) give presentations of Mars towards the sun at twelve periods of the Martial year,—viz., at the autumnal and vernal equinoxes, at the two solstices, and at intermediate periods corresponding to our terrestrial months.
In fact, by means of these projections one might readily form a series of sun-views of Mars resembling my 'Sun-views of the Earth.'
In the first view of Jupiter it is to be remarked that the three satellites outside the disc are supposed to be moving in directions appreciably parallel to the belts on the disc—the upper satellites from right to left, the lower one from left to right. In general the satellites, when so near to the disc, are not seen in a straight line, as the three shown in the figure happen to be. Of the three spots on the disc, the faintest is a satellite, the neighbouring dark spot its shadow, the other dark spot the shadow of the satellite close to the planet's disc.