And, referring to this same order of objects, M. Arago says:--"The forms of very large diffuse nebulæ do not appear to admit of definition; they have no regular outline." This coexistence of largeness, irregularity, and indefiniteness of outline, with irresolvability, is extremely significant. The fact that the largest nebulæ are either irresolvable or very difficult to resolve, might have been inferred _a priori_; seeing that irresolvability, implying that the aggregation of precipitated matter has gone on to but a small extent, will be found in nebulæ of wide diffusion. Again, the irregularity of these large, irresolvable nebulæ, might also have been expected; seeing that their outlines, compared by Arago with "the fantastic figures which characterize clouds carried away and tossed about by violent and often contrary winds," are similarly characteristic of a mass not yet gathered together by the mutual attraction of its parts. And once more, the fact that these large, irregular, irresolvable nebulæ have indefinite outlines--outlines that fade off insensibly into surrounding darkness--is one of like meaning. Speaking generally (and of course differences of distance negative anything beyond average statements), the spiral nebulæ are smaller than the irregular nebulæ, and more resolvable; at the same time that they are not so small as the regular nebulæ, and not so resolvable. This is as, according to the hypothesis, it should be. The degree of condensation causing spiral movement, is a degree of condensation also implying masses of flocculi that are larger, and therefore more visible, than those existing in an earlier stage. Moreover, the forms of these spiral nebulæ are quite in harmony with the explanation given. The curves of luminous matter which they exhibit, are _not_ such as would be described by discrete masses starting from a state of rest, and moving through a resisting medium to a common centre of gravity; but they _are_ such as would be described by masses having their movements modified by the rotation of the medium.