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which latter glass is no more likely to develop hygroscopic crystallisation than is the ordinary crown glass used for older type double objective.
We have occasionally had cases, however, where ordinary double objectives that have been used in damp climates have developed the same crystallisation on the interior surface of the crown lens.
Indeed, in damp tropical climates such as that of Calcutta the crystallisation which so frequently develops on the interior surfaces of ordinary crown glass lenses is found, if left too long a time without wiping off, to leave an actual corrosion of the surface in the form of a fine grey veil. The appearance is just like that shown by an imperfectly polished surface, in which the fine pittings caused by the last emeries still survive.
Messrs. Cooke & Sons do not now employ the crown glass O 374, as it wants cleaning more often than is desirable, and use the boro-silicate crown of type O 599, or else a variety of it recently recommended by Messrs. Schott. As regards freedom from secondary spectrum, it is difficult to detect any difference between objectives made of these two different crown glasses.
DESCRIPTION OF PLATES.
Figs. 1 and 2. Photographs of two 3-in. lenses after 83 months' use in a laboratory spectroscope. Fig. 1, collimating lens; fig. 2, camera lens.
Fig. 1. Photograph of 3'5-in. lens after 40 months' use as a finder to a 3-ft. reflector.
Fig. 2 is a reproduction of a photograph of the same lens, but taken so as to show the markings (white dots near centre) on another surface of the combination.
Figs. 1, 2, and 3. Enlarged views (700 diameters) of some of the white markings illustrated in Plate 2, fig. 2.
Figs. 4, 5, and 6. Enlarged views (45, 150, 700 diameters respectively) of parts of one of the groups of crystals shown on Plate 2, fig. 1.
The Nebula HIV 74 Cephei. By Dr. Max Wolf. (Plate 4.)
I have pointed out elsewhere that, as a rule, the nebulæ in the Milky Way are encircled by a ring which is void of faint stars, and that this lacuna is the end of a long starless hole, apparently showing the direction of some unknown cosmic motion. I have given a number of examples of this, the best specimen being the nebula 2 degrees south of 2 Cygni.
On photographing the Nebula H IV 74 Cephei (=N.G.C. 7023: a=21h 0m 308 8 +67° 46' 2 (1900'0)) with the 28-inch reflector of the Königstuhl Astrophysical Observatory, I was surprised to find a striking specimen of this phenomenon, the nebula being surrounded by a lacuna absolutely empty of faint stars. In spite of the small field of the reflector-plate, there was evidence that the cavity was the end of a channel entering the picture almost exactly from the south.
This nebula was photographed by the late Dr. Roberts (vol. ii., plate 24). He says: "The nebula appears in a region almost void of stars" (p. 157).
I then endeavoured to photograph the channel with the large field of my 16-inch cameras. But the weather was very unfavourable during this spring and summer, and the sky always so hazy, that three nights were lost in these attempts. The last of these trials, on July 7 (exposure 3 hours 22 minutes), gave relatively the best picture, a reproduction of which is given on Plate 4, fig. 1. The scale is about 70 mm.† for 1 degree. As may be seen, the channel is directed in its first part almost exactly from south to north. At a distance of about 1 degree from the centre of the nebula the channel is divided into two branches. The eastern branch is perhaps shorter, and the western longer, ending at a distance of about 1 degrees from the nebula by becoming gradually filled up with stars. The somewhat round cavity encircling the nebula has a diameter of about half a degree.
We have therefore a further example of the rule that a nebula is encircled by lacuna void of stars, which lacuna is the end of an empty chanuel.
The nebula itself is a very remarkable object. respects it resembles in form the 2 Cygni nebula. On the other hand, the faint bands of nebulosity parallel with the brighter bands are very much like those in the Orion nebula. This parallelism gives an indication of some kind of wave formation in the nebulous material. These are especially visible at the north-west side, three parallel waves, each nearly half the brightness of the inner preceding wave. I am especially struck by the gradual fading of the light of the nebulous matter from the centre, producing the impression that nebulous matter is spread all over
* Astron. Nachr., 3848; M.N., R. A.S., Ixiv., No. 9; Public. Astrophys. Inst., Königstuhl, Heidelberg, vols. i. and ii., etc.
In the reproduction 48 mm. = 1°.