adopted in order to enable all the work both of observing and of recording to be done by himself.  This, in many ways, was a great drawback to the work of the younger astronomer.  The division of labour between the observer and the scribe enables a greatly increased quantity of work to be got through.  It is also distinctly disadvantageous to an observer to have to use his eye at the telescope directly after he has been employing it for reading the graduations on a circle, by the light of a lamp, or for entering memoranda in a note book.  Nebulae, especially, are often so excessively faint that they can only be properly observed by an eye which is in that highly sensitive condition which is obtained by long continuance in darkness.  The frequent withdrawal of the eye from the dark field of the telescope, and the application of it to reading by artificial light, is very prejudicial to its use for the more delicate purpose.  John Herschel, no doubt, availed himself of every precaution to mitigate the ill effects of this inconvenience as much as possible, but it must have told upon his labours as compared with those of his father.

But nevertheless John Herschel did great work during his “sweeps.”  He was specially particular to note all the double stars which presented themselves to his observation.  Of course some little discretion must be allowed in deciding as to what degree of proximity in adjacent stars does actually bring them within the category of “double stars.”  Sir John set down all such objects as seemed to him likely to be of interest, and the results of his discoveries in this branch of astronomy amount to some thousands.  Six or seven great memoirs in the transactions of the Royal Astronomical Society have been devoted to giving an account of his labours in this department of astronomy.

[Plate:  The cluster in the Centaur, drawn by Sir John Herschel.]

One of the achievements by which Sir John Herschel is best known is his invention of a method by which the orbits of binary stars could be determined.  It will be observed that when one star revolves around another in consequence of the law of gravitation, the orbit described must be an ellipse.  This ellipse, however, generally speaking, appears to us more or less foreshortened, for it is easily seen that only under highly exceptional circumstances would the plane in which the stars move happen to be directly square to the line of view.  It therefore follows that what we observe is not exactly the track of one star around the other; it is rather the projection of that track as seen on the surface of the sky.  Now it is remarkable that this apparent path is still an ellipse.  Herschel contrived a very ingenious and simple method by which he could discover from the observations the size and position of the ellipse in which the revolution actually takes place.  He showed how, from the study of the apparent orbit of