superfluous ought to be ground off, to give passage for as much light as is possible from the object to the speculum.

One great advantage of this prism, which cannot be obtained from the oval metal, is, that without using two glasses the object may be erected, and the magnifying power of the telescope varied at pleasure, by merely varying the distances of the speculum, the prism, and the eye-glass. This will be understood from fig. 16, where AI represents Fig. 16.

the great concave speculum, EF the eye-glass, and BCD the prism of glass, whose sides BC and CD are not flat, but spherically convex. The rays which come from G, the focus of the great speculum AI, will, by the refraction of the first side BC, be reduced to parallelism, and after reflection from the base CD, will be made by the refraction of the next side BD to converge to the focus H of the eye-glass EF. If we now bring the prism BCD nearer the image at G, the point H will recede from BD, and the image formed there will be greater than that at G, and if we remove the prism BCD from G, the point H will approach to BD, and the image at H

will be less than that at G. The prism BCD performs the same part as a convex lens, G and H being its conjugate foci, and the relative size of the images formed at these points being proportional to their distance from the lens. This construction would be a good one for varying optically the angular distance of a pair of wires placed in the focus of the eye-glass EF; and by bisecting the lenticular prism BCD, and giving the halves a slight inclination, we should be able to separate and to close the two images or disks which would thus be produced, and thus form a double image micrometer.

Among the minor and detached labours of Sir Isaac, we must not omit his curious experiments on the action of light upon the retina. Locke seems to have wished his opinion respecting a fact stated in Boyle's Book on Colours, and in a letter from Cambridge, dated June 30th, 1691, he communicated to his friend the following very remarkable observations made by himself.

"The observation you mention in Mr. Boyle's book of colours I once made upon myself with the hazard of my eyes. The manner was this; I looked a very little while upon the sun in the looking-glass with my right eye, and then turned my eyes into a dark corner of my chamber, and winked, to observe the impression made, and the circles of colours which encompassed it, and how they decayed by degrees, and at last vanished. This I repeated a second and a third time. At the third time, when the phantasm of light and colours about it were almost vanished, intending my fancy upon them to see their last appearance, I found, to my amazement, that they began to return, and by little and little to become as lively and vivid as when I had newly looked upon the sun. But when I ceased to intend my fancy upon them, they vanished again. After this, I found, that as often as I went into the dark, and intended my mind upon them, as when

a man looks earnestly to see any thing which is difficult to be seen, I could make the phantasm return without looking any more upon the sun; and the oftener I made it return, the more easily I could make it return again. And at length, by repeating this without looking any more upon the sun, I made such an impression on my eye, that, if I looked upon the clouds, or a book, or any bright object, I saw upon it a round bright spot of light like the sun, and, which is still stranger, though I looked upon the sun with my right eye only, and not with my left, yet my fancy began to make an impression upon my left eye, as well as upon my right. For if I shut my right eye, or looked upon a book or the clouds with my left eye, I could see the spectrum of the sun almost as plain as with my right eye, if I did but intend my fancy a little while upon it; for at first, if I shut my right eye, and looked with my left, the spectrum of the sun did not appear till I intended my fancy upon it; but by repeating, this appeared every time more easily. And now, in a few hours' time, I had brought my eyes to such a pass, that I could look upon no bright object with either eye but I saw the sun before me, so that I durst neither write nor read; but to recover the use of my eyes, shut myself up in my chamber made dark, for three days together, and used all means to divert my imagination from the sun. For if I thought upon him, I presently saw his picture, though I was in the dark. But by keeping in the dark, and employing my mind about other things, I began in three or four days to have some use of my eyes again; and, by forbearing to look upon bright objects, recovered them pretty well, though not so well but that, for some months after, the spectrum of the sun began to return as often as I began to meditate upon the phenomena, even though I lay in bed at midnight with my curtains drawn. But now I have been very well for many years, though I am

apt to think, if I durst venture my eyes, I could still make the phantasm return by the power of my fancy. This story I tell you, to let you understand, that in the observation related by Mr. Boyle, the man's fancy probably concurred with the impression made by the sun's light to produce that phantasm of the sun which he constantly saw in bright objects. And so your question about the cause of this phantasm involves another about the power of fancy, which I must confess is too hard a knot for me to untie. To place this effect in a constant motion is hard, because the sun ought then to appear perpetually. It seems rather to consist in a disposition of the sensorium to move the imagination strongly, and to be easily moved, both by the imagination and by the light, as often as bright objects are looked upon."

These observations possess in many respects a high degree of interest. The fact of the transmission of the impression from the retina of the one eye to that of the other is particularly important; and it deserves to be remarked, as a singular coincidence, that I had occasion to observe and to describe the same phenomena above twenty years ago,* and long before the observations of Sir Isaac were communicated to the scientific world.

*Art. Accidental Colours in the Edinburgh Encyclopædia.

CHAPTER XVIII.

His Acquaintance with Dr. Pemberton, who edits the Third Edition of the Principia-His first Attack of ill Health-His Recovery- He is taken ill in consequence of attending the Royal Society-His Death on the 20th March, 1727-His Body lies in state-His Funeral-He is buried in Westminster Abbey-His Monument described-His Epitaph-A Medal struck in honour of him-Roubiliac's full-length Statue of him erected in Cambridge-Division of his Property-His Successors.

ABOUT the year 1722, Sir Isaac was desirous of publishing a third edition of his Principia, and the premature death of Mr. Cotes having deprived him of his valuable aid, he had the good fortune to become acquainted with Dr. Henry Pemberton, a young and accomplished physician, who had cultivated mathematical learning with considerable success. M. Poleni, an eminent professor in the University of Padua, having endeavoured, on the authority of a new experiment, to overturn the common opinion respecting the force of bodies in motion, and to establish that of Leibnitz in its place, Dr. Pemberton transmitted to Dr. Mead a demonstration of its inaccuracy. Dr. Mead communicated this paper to Sir Isaac, who not only highly approved of it, but added a demonstration of his own, drawn from another consideration of the subject; and this was printed without his name, as a postscript to Pemberton's paper, when it appeared in the Transactions.*

In a short time after the commencement of their acquaintance, Sir Isaac engaged Dr. Pemberton to superintend the new edition of the Principia. In discharging this duty, Dr. Pemberton had occasion to make many remarks on this work, which Sir Isaac

* See Phil. Trans, 1722, vol. xxxiii. p. 57.