Dr. Max Wolf has discovered a number of small nebulæ in the regions near Algol and Nova Persei (the great "new star" of 1901). He says, "They mostly lie in two bands," and are especially numerous where the two bands meet, a region of 12 minutes of arc square containing no less than 148 of them. They are usually "round with central condensation," and form of Andromeda nebula.[353] Some small nebulæ have been found in the vicinity of the globular clusters. They are described by Prof. Perrine as very small and like an "out of focus" image of a small star. "They appear to be most numerous about clusters which are farthest from the galaxy." Prof. Perrine says, "Practically all the small nebulæ about the globular clusters are elliptical or circular. Those large enough to show structure are spirals. Doubtless the majority of these are spirals."[354] This seems further evidence in favour of the "spiral nebular hypothesis" of Chamberlin and Moulton. A great photographic nebula in Orion was discovered by Prof. Barnard in 1894. In a drawing he gives of the nebula,[355] it forms a long streak beginning a little south of γ Orionis (Bellatrix), passing through the star 38 Orionis north of 51 and south of 56 and 60 Orionis. Then turning south it sweeps round a little north of κ Orionis; then over 29 Orionis, and ends a little to the west of η Orionis. There is an outside patch west of Rigel. Barnard thinks that the whole forms a vast spiral structure; probably connected with the "great nebula" in the "sword of Orion," which it surrounds. From calculations of the brightness of surface ("intrinsic brightness") of several "planetary" nebulæ made by the present writer in the year 1905, he finds that the luminosity is very small compared with that of the moon. The brightest of those examined (_h_ 3365, in the southern hemisphere, near the Southern Cross) has a surface luminosity of only 1/400 of that of the moon.[356] The great nebulæ in Orion and Andromeda seem to have "still smaller intrinsic brightness." Arago says-- "The spaces which precede or which follow simple nebulæ, and _a fortiori_ groups of nebulæ, contain generally few stars. Herschel found this rule to be invariable. Thus every time that, during a short interval, no star appeared, in virtue of the diurnal motion, to place itself in the field of his motionless telescope, he was accustomed to say to the secretary who assisted him (Miss Caroline Herschel), 'Prepare to write; nebulæ are about to arrive.'"[357] Commenting on this remark of Arago, the late Herbert Spencer says--