The spots may often be easily observed with telescopes of small dimensions, taking care, however, to place in front of the eye-piece a piece of strongly-coloured glass. For this purpose glasses of various colours are used, but none so good as dark green or dark neutral tint. It is not altogether easy to say positively how large a spot must be for it to be visible with the naked eye, or an opera glass, but probably it may be taken generally that no spot of lesser diameter than 1′ of arc can be so seen. This measurement must be deemed to apply to that central portion of a normal spot already mentioned as being what is called the nucleus, because penumbræ may be more than 1′ in diameter without being visible to the naked eye, for the reason that their shading is so much less pronounced than the shading of umbræ. Very large and conspicuous spots are comparatively rare, though during the years 1893 and 1894 there were an unusual number of such spots. It often happens that a conspicuous group is the result of the merging or joining up of several smaller groups. In such cases a group may extend over an area on the Sun 3′ or 4′ of arc in length by 2′ or 3′ in breadth. The largest spot on record seems to have been one seen on September 30, 1858, the length of which in one direction amounted to more than 140,000 miles. The observation of spots on the Sun by projecting them on to a white paper screen with the aid of a telescope is a method so convenient and so exact as to deserve a detailed description, the more so as it is so little used. Let there be made in the shutter of a darkened room a hole so much larger than the diameter of the telescope to be used as will allow a certain amount of play to the telescope tube, backwards and forwards, up and down, and from right to left. Direct the telescope to the Sun and draw out the eye-piece to such a distance from the object-glass as that the image projected on a white screen held behind may be sharply defined at its edges. If there are any spots on the Sun at the time they will then be seen clearly exhibited on the screen. An image obtained in this way is reversed as compared with the image seen by looking at the Sun through a telescope directly. If therefore the telescope is armed with the ordinary astronomical eye-piece, which inverts, then the projection will be direct, that is to say, on the screen the N. S. E. and W. points will correspond with the same terrestrial points. Under such circumstances the spots will be seen to enter the Sun’s disc on the E. side and to go off on the W. side. The contrary condition of things would arise if a Galilean telescope or a terrestrial telescope of any kind were made use of. These instruments erect the image, and therefore will give by projection a reversed image, in which we shall see the spots moving apparently in a direction contrary to their true direction.