compared with the wedge. In this way the result is attained. that the two surfaces to be compared are observed under perfectly identical conditions. The measurements with this apparatus are accordingly made in the following manner: after an appropriate wedge has been placed in the slide and the illumination regulated so that the ground-glass disk R is perpendicularly illuminated, the observer first focuses the eyepiece sharply on the edge of the small mirror in the double prism, draws the tube F out to the frame M, and focuses on the grain of the wedge with the pinion E. The plate to be measured is then laid upon the table, and the observer focuses upon it with the pinion H and slides the tube K close up to the plate. The measurements can then begin. The plate is then moved upon the table in such a way that the first field of the scale on the plate appears in the small mirror, and the wedge is moved by turning the head P until this place in the middle of the prism has exactly the same appearance as its surroundings. The equalization of the two surfaces can be accurately accomplished and sharply determined, since the dividing line disappears entirely at the correct setting. The setting of the scale is then read off and the next field of the scale is taken up. A series of measurements thus obtained then furnishes a table of the settings of the wedge which correspond to the several fields of the scale and accordingly to the known brightnesses of this plate. After this the picture taken on the plate is measured in the same way and the readings of the wedge thus obtained serve for interpolating from the table the brightness corresponding to the degree of blackening in units of the lamp employed. I have only to refer to another attachment intended both for rapidly finding the place to be observed in the picture and for making a preliminary test of the plate. If the area of blackening which is to be measured is very small and situated in an irregular area of different brightness, it would be difficult to set the desired portion of the plate exactly at the reflecting surface in the middle of the prism, since all of the surrounding parts of the plate cannot be seen in the eyepiece, so that the orientation is entirely lost. In order to render possible a rapid and exact setting of the desired object in this case, the double prism B C can be removed by pressure on a button, and in its place a simple reflecting prism, similar to B, can be brought up, which permits the greater part of the plate to be seen. A curve drawn on the base of this prism and corresponding to the outline of the reflecting surface in the double prism then bounds that part of the plate in the field of view which alone can be observed after replacing the double prism. The simple prism and double prism are attached beside each other in a slide which can be moved back and forth between stops, so that a change of prisms can be made in less than a second. In Fig. I the slide may be seen pushed out to its right hand stop. In the form thus described the apparatus serves for the direct comparison of the density of the silver precipitation, since the magnification employed clearly shows the grain of the more sensitive bromide of silver plates. If the grain of the two plates to be compared is very different, however, as may be the case in sensitometer measurements, it is better to compare the transparency of the two plates. For this purpose we may either screw on in place of the objectives D and G, two equal diaphragms furnished with the apparatus, which may be brought close to the film, as is sufficient in many cases, or we may disturb the focus by the pinions E H until the grain disappears. Measurement with a sharp focus is to be preferred in general, since it is here easy to recognize with the use of the small prism, whether the area focused upon is uniformly blackened or whether its transparency is affected by specks or holes in the film. Measurements can be made with extreme rapidity and ease with this apparatus, and I beg to give an example of their accu racy. Each of three observers made ten settings on one and the same portion of a scale and obtained the following readings of the setting of the wedge. In order to determine to what range of brightness the displacement of the wedge by 1 mm corresponds at the portion here used, the two neighboring fields of the scale, the coefficients of absorption of which had been previously determined, were also compared, with the following result : Log I is the logarithm of the quantity of light transmitted by the particular portion of the scale, the quantity of incident. light being called unity. C is the range of log I for displacement of the wedge by 1 mm. If we place C=-0.0365 as a mean value, for the portion of the wedge used above, I mm of the wedge will here correspond to a range of brightness of 8.77 per cent., or, expressed astronomically, to 0.091 magnitudes. The differences between the above results of the three observers are accordingly: THE GREAT SUN-SPOT OF SEPTEMBER 1898. By J. FÉNYI, S. J. THIS phenomenon, remarkable enough on account of its enormous size, receives a special significance from the very unusual aurora which accompanied it. Even at Kalocsa, in latitude 46° 32′ and longitude 18°58' east of Greenwich, the aurora was observed for about a quarter of an hour at 9h Gr. M. T. on September 9. An aurora is here an almost unheard of occurrence, one like this not having been seen in this section, even at a solar maximum, since 1872. It therefore does not seem superfluous to communicate the observations of the spot rade at Kalocsa. I made drawings of the spot on twelve days of its transit. The accompanying drawing was at 10 A.M. Gr. M. T. on September 5, with the helioscope attached to the 41⁄2-inch telescope. From the whole number of observations with projection apparatus I determined the heliographic longitude as 241.7°, and the latitude as -12.5°. According to these determinations the spot passed around the east limb of the disk on September 2 at 8h A. M.; when I made the regular observations of spots with the projection apparatus at 1h 45m the spot was not yet visible, but as I showed the Sun to a visitor at 3h 45m the strong dark line. on the eastern edge at once attracted attention. According to computation the spot was then at a distance of 3.3" from the edge of the disk, without regarding refraction. I at once applied the spectroscope and now found the place, which at the regular observations at 1h 30m P. M. showed nothing unusual, to be already in an eruptive state. The eruptive line at A6677 was visible for an extent of 11°, and from position angle 119° 30' to 1210' the chromosphere appeared extremely bright and covered with the familiar flames. This point, however, did not lie over the nucleus of the spot indicated by the dark strip, although very near to it, being about one half degree nearer to |