A comparison of these several representations of the same thing will serve to illustrate the vagueness of its outlines, and how much the impressions to be derived from nebulæ depend upon the telescopes employed and upon the observer's own prepossessions. The differences among the pictures can not be due to any change in the nebula itself, for half a century ago it was sketched much as shown in the latest of them (Fig. 140). [Illustration: FIG. 142.--The Trifid nebula.--KEELER.] 215. TYPICAL NEBULÆ.--Some of the fantastic forms which nebulæ present in the telescope are shown on a small scale in Fig. 141, but in recent years astronomers have learned to place little reliance upon drawings such as these, which are now almost entirely supplanted by photographs made with long exposures in powerful telescopes. One of the most exquisite of these modern photographs is that of the Trifid nebula in Sagittarius (Fig. 142). Note especially the dark lanes that give to this nebula its name, Trifid, and which run through its brightest parts, breaking it into seemingly independent sections. The area of the sky shown in this cut is about 15 per cent less than that covered by the full moon. [Illustration: FIG. 143.--A nebula in Cygnus.--KEELER.] Fig. 143 shows a very different type of nebula, found in the constellation Cygnus, which appears made up of filaments closely intertwined, and stretches across the sky for a distance considerably greater than the moon's diameter. [Illustration: FIG. 144.--Spiral nebula in Canes Venatici.--KEELER.] A much smaller but equally striking nebula is that in the constellation Canes Venatici (Fig. 144), which shows a most extraordinary spiral structure, as if the stars composing it were flowing in along curved lines toward a center of condensation. The diameter of the circular part of this nebula, omitting the projection toward the bottom of the picture, is about five minutes of arc, a sixth part of the diameter of the moon, and its thickness is probably very small compared with its breadth, perhaps not much exceeding the width of the spiral streams which compose it. Note how the bright stars that appear within the area of this nebula fall on the streams of nebulous matter as if they were part of them. This characteristic grouping of the stars, which is followed in many other nebulæ, shows that they are really part and parcel of the nebula and not merely on line with it. Fig. 145 shows how a great nebula is associated with the star ρ Ophiuchi. [Illustration: FIG. 145.--Great nebula about the star ρ Ophiuchi.--BARNARD.]