The Conference had a number of very important questions to discuss, for knowledge of the photographic method and its possibilities was at that time in its infancy. There was, for instance, the question whether all the instruments need be of the same pattern, and if so what that pattern should be. The first of these questions was settled in the affirmative, as we might expect; in the interests of uniformity it was desirable that the maps should be as nearly similar as possible. The second question was not so easy; there were at least three different types of instruments which might be used. First of all, there was the photographic lens, such as is familiar to all who have used an ordinary camera, consisting of two lenses with a space between; though since each of these lenses is itself made up of two, we should more correctly say four lenses in all. It was with a lens of this kind that the comet pictures had been taken at the Cape of Good Hope, and it might seem the safest plan to adopt what had been shown to be capable of such good work. But there was this difficulty; the pictures of the comet were on a very small scale, and taken with a small lens; a much larger lens was required for the scheme now under contemplation, and when there are four separate lenses to be made, each with two surfaces to polish, and each requiring a perfectly sound clear piece of glass, it will be obvious that the difficulties of making a large compound lens of this kind are much greater, and the expense much more serious than in the case of a single lens, or even a pair. It was this question of expense which had led the brothers Henry to experiment with a different kind of instrument, in which only one pair of lenses was used instead of two. Their instrument was, in fact, very similar to the ordinary telescope, excepting that they were bound to make their lenses somewhat different in shape in order to bring to focus the rays of light suitable for photography, which are not the same as those suitable for eye observation with the ordinary telescope. Dr. Common, again, had used a third kind of instrument, mainly with the view of reducing the necessary expense still further, or, perhaps, of increasing the size of the instrument for the same expense. His telescope had no lens at all, but a curved mirror instead, the mirror being made of glass silvered on the face (not on the back as in the ordinary looking-glass). In this case there is only one surface to polish instead of four, as in the Henrys' telescope, or eight, as in the case of the photographic doublet; and, moreover, since the rays of light are reflected from the surface of the glass, and do not pass _through_ it at all, the internal structure of the glass is not so strictly important as in the other cases. Hence the reflector is a very cheap instrument, and it is, moreover, quite free from some difficulties attached to the other instruments. No correction for rays of light of