condensations, and that their form is determined by the velocity of rotation. The fact that the singular configurations of the central zone, though lying in the region of the greatest speed, showed but little trace of the general parallelism, may be explained on the supposition that they lay too deep for this disturbing force, which may naturally be most active towards the upper limit of the atmosphere. To this, however, it might be objected that, if darkness is to be held a test of depth, the north. temperate belt, at no great distance, gave testimony of an action penetrating to a much lower region than that of those central markings, whose comparative quiescence thus remains unaccounted for; and the matter seems to require further investigation.

As regards the decided colouring of the equatorial zone, an idea suggests itself of two distinct strata of clouds, of which the lower reflect a yellower, the upper a whiter light; or the difference might have been ascribed to different thicknesses of a yellow atmosphere, but that on this supposition the tint should have grown deeper towards the limbs, which I do not think was the case. If we may conjecture such a double arrangement, it would follow that the slight variation in warmth of the planet's seasons is not very directly concerned in the distribution of these clouds, as the north or summer hemisphere possessed at once the whitest of the luminous zones, and in its grey clearings showed a yellower tinge; to which may be added what the observations appear to prove, that the coloured zone maintained an equatorial position, notwithstanding the departure of the sun from its zenith. We may, however, suppose, from the comparative transparency of the polar skies, that solar heat in general is concerned in the production of these clouds. Sufficient attention has, perhaps, not been drawn to the perspective of these atmospheric obscurations. In any oblique presentation of the luminous zones, their thickness, which we cannot imagine to be insensible, would so come into view as optically to diminish the open intervals between them; and this diminution would be progressive with the approach to the pole; so that we should have the appearance of such a continuous brightness in the arctic regions as does not seem to have been matter of observation; and hence we may infer a comparative absence of condensed vapour in those parts of the globe.

I have not been able to corroborate the general assertion that the grey belts become much lighter towards their ends. I have repeatedly remarked that they faded but little towards the limb; and, in the fine observation of Nov. 17, I have noted that I hardly thought the difference would have struck me if I had not looked for it: the principal belts could then be

readily followed quite to the limb, as in De la Rue's magnificent engraving. Nothing was ever witnessed at all approaching to the obliteration figured in Beer and Mädler's "Beiträge; " nor did the spots disappear, as those authorities found, at distances of 52° and 54° from the central meridian, though they were much more difficult objects towards the limb. It is very conceivable that the planet's atmosphere may vary greatly in different seasons, and much must be allowed for the superior power of my instrument. The effect of perspective has, however, perhaps not been always borne sufficiently in mind. A marking whose visibility depends upon its breadth will disappear from simple foreshortening in the neighbourhood of the limb, while a belt extended in the opposite direction will experience no obliteration from that cause.

The peculiar uniformity of aspect in the central region would probably be much broken up, as we know to be the case upon the Moon, by the application of greater optical power; nevertheless, enough of regularity must remain to indicate the action. of some widely extended but unknown law. The very limited access which I at present possess to the necessary materials for comparison induces me to postpone an enquiry into the occasional, if not frequent, recurrence of this remarkable phenomenon; and it may well be taken up by more competent hands. Such recurrence of configuration, however, and that not confined solely to the region of the equator, may be at once indicated as matter of the highest probability. Very marvellous arrangements evidently exist upon that planet, and forces are in operation, of the nature of which we can at present form little other idea, excepting that they must declare the power of the Great Creator upon a scale of surpassing magnitude.

MICROSCOPIC TEST OBJECTS UNDER PARALLEL

LIGHT AND CORRECTED POWERS.

BY THE REV. J. B. READE, F.R.S., P.R.M.S.

[PLATE LVIII.]

TH

INTRODUCTION.

THE characters of objects we can neither see nor touch can be revealed by the microscope only. We must have a magnifying apparatus and an illuminating apparatus. The former, the magnifying apparatus, i.e. the whole battery of powers from 1th to th of an inch, may be readily obtained from Messrs. Ross, Wray, Beck, and Powell; and these eminent English opticians have spared neither labour nor expense in working up to the most elaborate formula. The latter, the illuminating apparatus, has much wider scope-is untrammelled by definite rules of construction, and brought out under every variety of shape. A "condenser " is what we want, i.e. an instrument for giving a proper angle to a suitable pencil of light. But what a list we have to choose from! Wollaston, Brewster, Shadbolt, Wenham, Nobert, Amici, Gillett, Kingsley, Dujardin, Reade, cum multis aliis, are in the field, each one offering some peculiarity of construction or some special principle of illumination. Hence the optician himself is nonplussed here, and he can only say to his customer, in the language of the showman, "Pay your money, and take your choice."

In attempting to make the microscope perfect as an instrument of research a universal testimony is borne to the importance of illumination; and it is not too much to say that illumination is the soul of the complex body, with all its ingenious mechanism, appliances, and powers. In dealing with the problem of illumination a few years ago, I felt convinced that in all our condensers a most important desideratum was wanting. We had fixed apertures for all the powers and for all, even the