208 



SCIENCE. 



[N. S. Vol. XII. No. 293. 



ring series of impacts, of the smaller neb- 

 ula. So far as we can now see, the most 

 serious difficulty in framing a consistent 

 hypothesis along this line lies in the ap- 

 proximate circularity of the present plane- 

 tary orbits, but as circularity may result 

 from the combination of a large number of 

 constituents having elliptical orbits, this 

 difBculty may not prove insuperable. 



"We naturally turn to the heavens for 

 nebulse whose evolution might give a sys- 

 tem of low mass and high momentum in 

 the outer part and high mass and low mo- 

 mentum in the central part. The spiral 

 nebulae offer the greatest promise of con- 

 forming to these demands for they seem to 

 present attenuated outer matter irregularly 

 dispersed and perhaps in relatively high 

 motion, while the central portions are usu- 

 ally denser and seem to possess less momen- 

 tum relatively, but this is little more than 

 pure conjecture based on their forms, for 

 nothing is positively known of the dynamics 

 of these masses. Professor Keeler has shown 

 by recent photographic researches that 

 spiral nebulae are the dominant forms among 

 the smaller class. This justifies us in giv- 

 ing them precedence in attempts to find 

 analogies for the origin of our system. This 

 suggestion may really be identical with the 

 preceding, for, in the absence of any knowl- 

 edge of the origin of spiral nebulse, it is 

 possible to conjecture that they arose from 

 peripheral collisions of antecedent nebulse. 

 T. C. Chamberlin, 

 f. e.. moulton. 



Univeesity of Chicago, 

 July 9, 1900. 



THE ILLUSORY BUST DRIFT. A CURIOUS 

 OPTICAL PHEN03IEN0N: 

 It is of course improbable in the highest 

 degree that the phenomenon here to be 

 described has entirely escaped notice hith- 

 erto, but the writer at least is unaware of 

 any existing description of the same. The 



conditions under which the illusion arises 

 are so easy to fulfill, and the resulting 

 appearance is so odd in many ways, that 

 the readers of Science may be interested in 

 a brief description of the matter. The only 

 ' apparatus ' required is a set of black and 

 white lines and a dark background near by. 

 The best results, perhaps, are obtained by 

 using a square yard of common black cloth 

 bearing narrow white lines not more than 

 two millimeters apart. Such cloth may be 

 obtained at any large dry goods store. If 

 now this be hung upon the wall in a strong 

 light, and a square of dull black cardboard 

 be placed above it, or at the side, every- 

 thing is ready for the observation. Picking 

 out some point near the center of the cloth, 

 let this be fixated steadily for not less than 

 twenty seconds. Then transfer the gaze 

 quickly to the black cardboard, and the 

 illusory dust drift will appear. The appear- 

 ance is that of a thin cloud of fine ivMte dust 

 moving across the field of vision. Or the tiny 

 particles seen may be likened to the motes 

 in a sunbeam, since they much resemble 

 these in density. A steady fixation of the 

 eyes is at no time absolutely essential. 

 They may roam freely over the cloth and 

 then later over the dark background, though 

 the illusion under these circumstances is 

 diminished in strength. The best results 

 are unquestionably secured by as resolute 

 a fixation of the cloth as possible. The 

 necessary duration of this fixation seems to 

 depend upon the retinal sensitiveness of the 

 observer. Probably 5 sec. is the minimum 

 for any noticeable after effects, while no 

 advantage seems to be gained in any direc- 

 tion by prolonging the fixation beyond a 

 period of 30 sec. In practice, successive 

 renewals of the illusion may be accomplished 

 by very brief fixations, provided only that 

 the time of the first fixation be moderately 

 long. 



The duration of the illusion seems also to 

 be an individual matter. One observer can 



