188 Nebula. 



2°*44. It seems then to follow that either the nebula (together 

 with its stellar centre), or its companion star, must have moved 

 very sensibly during the last thirty years ; and this conclusion 

 is confirmed by Mr. Knott's reduction of Argelander's inter- 

 mediate places. It is impossible at present to say which of 

 the two has changed its position, but this will be demonstrated 

 by future measures ; and there is a very minute star y, forming 

 the apex of a triangle with the others, which, as Mr Knott 

 has remarked, would form a good point of reference, if it has 

 sufficient brightness for measurement. Since Mr. Huggins's 

 wonderful discovery, we are more prepared to accept the 

 alternative of motion in either ; and the nebulosity, if it pos- 

 sesses light enough, will be a most interesting object for the 

 spectroscope. If it were not waste of time to reconcile so 

 rough a guess as mine with the results of admirable micro- 

 metric measurement, it might be observed that they would be 

 very closely approximated by allowing for the extra-polar 

 position of Polaris, and for a remarkable bias in vision with 

 which I have since become acquainted through the writings 

 of Sir J. Herschel. That eminent philosopher, in discussing 

 some ancient and valuable observations of Castor by Bradley 

 and Pound, in which the telescopic position of the components 

 was found to be parallel to the lines joining certain other stars 

 as seen with the naked- eye, found that angles thus estimated 

 required a correction for a tendency in the two eyes, when 

 comparing two straight lines, or two parts of the same straight 

 line, beheld by each separately , to give them a different posi- 

 tion with regard to the vertical. The difference amounted in 

 his own experience to 2° 43', at which angle lines really parallel 

 would seem to converge upwards, if so viewed, that while 

 both hues are looked at simultaneously, each eye sees only the 

 one opposite to itself. The experiment is easily tried with 

 any kind of thin screen edgeways in front of the face, but 

 requires a little care in manipulation ; for if the eye is very 

 unequally distant from the opposite extremities of the parallel 

 lines, the deviation will be reversed ; nor have I been able to 

 bend a horizontal line in the middle, as described by Herschel, 

 till it is placed in an oblique position. The following simple 

 mode of conducting the experiment, I have found effectual. 

 Take a large thin book, the thinner the better, provided it can 

 support itself without being opened, and set it on end upon a 

 table. On each side of it, pretty close to it, but not touching, 

 lay a sheet of white paper on the table, with a weight on it, so 

 that it may not easily be displaced ; then rest the nose on the 

 top of the book, when, of course, only one sheet will be visible 

 to either eye. While using both eyes arrange the sheets care- 

 fully, so that their nearest edges shall be as sensibly parallel 



