The circumstances which accompany the formation of a spot cannot readily be specified with certainty. It is impossible to say that there exists any law as to this matter. Whilst some spots develope very slowly by the expansion of certain pores, others spring into existence quite suddenly. Yet it cannot be said that the formation of a spot is ever completely instantaneous however rapid it may be. The phenomenon is often announced some days in advance: we may perceive in the photosphere a great agitation which often manifests itself by some very brilliant _faculæ_ (to be described presently) giving birth to one or more pores. Very often we next notice some groups of little black spots, as if the luminous stratum was becoming thinner in such a way as to disappear little by little and leave a large black nucleus uncovered. At the commencement of the business there is usually no clearly defined penumbra. This developes itself gradually and acquires a regular outline, just as the spot itself often takes a somewhat circular form. This tranquil and peaceable formation of a spot only happens at a time when calm seems to reign in the solar atmosphere: in general the development is more tumultuous and the stages more complicated. As a rule a spot passes through three stages of existence:—(1) the Period of birth; (2) a Period of calm; and (3) the Period of dissolution. When a spot is on the point of closing up, the flow of the luminous matter which it, as it were, attracts, is not directed uniformly towards the centre; it seems that the photospheric masses, no longer meeting with resistance, are precipitated promiscuously anywhere so as to fill up the hole. It is impossible to describe in detail the phases which irregular spots go through, but two things may always be remarked: that their structure is characterized by the existence of luminous filaments, and that these filaments converge towards one or several centres. Secchi thus sums up certain conclusions which he arrived at relating to spots generally:—(1) It is not on the surface of any solid body that the solar spots are manifested; they are produced in a fluid mass, the fluidity of which is represented by a gas, so that the constitution of this medium may be likened to that of flames or clouds; (2) the known details respecting the constitution of the penumbra and the phenomena exhibited prove that the penumbra is not a mass of obscure matter which floats across luminous matter, but that it is on the contrary a case of luminous matter invading and floating about over darker materials and so producing a half tint.