The largely-increased aperture which Dr. Janssen has given to his instrument is a point of great importance. In the early days of solar photography the aperture used was small, in order to prevent over-exposure. It was soon found that this small aperture, as was to be expected, produced poor images in consequence of the diffraction effects brought about by it. It then became a question of increasing the aperture while the exposure was reduced, and many forms of instantaneous shutters have been suggested with this end in view. With these, if a spring be used, the narrow slit which flashes across the beam to pay the light out into the plate changes its velocity during its passage as the tension of the spring changes. Of this again Dr. Janssen has not been unmindful, and he has invented a contrivance in which the velocity is constant during the whole length of run of the shutter. By these various arrangements the plates have now been produced at Meudon of fifteen inches diameter, showing details on the sun’s surface subtending an angle of less than one second of arc. So much for the _modus operandi_. Now for the branch of solar work which has been advanced. It is more than fifteen years ago since the question of the minute structure of the solar photosphere was one of the questions of the day. The so-called “mottling” had long been observed. The keen-eyed Dawes had pointed out the thatch-like formation of the penumbra of spots, when one day Mr. Nasmyth announced the discovery that the whole sun was covered with objects resembling willow-leaves, most strangely and effectively interlaced. We may sum up the work of many careful observers since that time by stating that the mottling on the sun’s surface is due to dome-like masses, and that the “thatch” of the penumbra is due to these dome-like masses being drawn, either directly or in the manner of a cyclone, towards the centre of the spot. In fact the “pores” in the interval between the domes are so many small spots, while the faculæ are the higher levels of the cloudy surface. The fact that faculæ are so much better seen near the limb proves that the absorption of the solar atmosphere rapidly changes between the levels reached by the upper faculæ and the pores. Thus much premised, we now come to Dr. Janssen’s discovery. An attentive examination of his photographs shows that the surface of the photosphere has not a constitution uniform in all its parts, _but that it is divided into a series of figures more or less distant from each other, and presenting a peculiar constitution_. These figures have contours more or less rounded, often very rectilinear, and generally resembling polygons. The dimensions of these figures are very variable; they attain sometimes a minute and more in diameter.