tions by it, though I have viewed the moon with it, and find it does not perform much worse, if any, than it did. Desiring to hear from you at your leisure, I rest your affectionate and obliged friend,

JOHN FLAMSTEED.

When Mr. Sargeant comes to London I shall send money, and directions for procuring me a new object glass, and beg your care or Mr. Hooke's in it. This letter is printed in the Gen. Dict. vol. V. p. 254.

CLXXIII.

FLAMSTEED TO COLLINS.

Derby, April 17, 1672.

Mr. Collins,

I have yours of the 12th instant, whereby you intimate your desires to have me say something further concerning the lunar theory of Mr. Horrox, for the satisfaction of Dr. Wallis. I am content; and therefore shall ere long send you the whole calculation of the moon's place at her last transit over the Pleiades, after his method, with my further thoughts upon it. I am very desirous to be framing lunar numbers, but want observations of her apogaon diameters, which the inconveniency of my dwelling permits me not to make, because the apogæon moon in Sagittarius goes so very low this year; and further, the refractions in her low meridional heights would greatly diminish her vertical diameters, which, upon continual experience, I find can only be taken to the requisite exactness; so that I fear I must wait till the year 1674, or 1675, ere I can well observe them. I cannot but much approve of the

form of Mr. Horrox's theory, and having considered it several times, I find that we might almost as easily compute her place in her orb, as the ecliptical place of any other planet, if tables were fitted to his method; which, therefore, for facility of calculation, I intend to propose to myself to correct when I shall gain so much leisure, and I hope this summer may afford it me.

I have perused Mr. Newton's letter concerning colours, but cannot think them to proceed from difform rays, capable of different refractions. For then no particular colour, but a mixture of all sorts, should be produced by the prism. For let four rays of light fall parallel to each other into the prism perpendicular to the first face, so that they may not be refracted till they pass forth from the second: of these we suppose the two outermost, E and H,° to be of that sort, which

are least refracted and make the red, they, therefore, passing through the prism, in their emersion move parallel to each other, and will paint the red at R' and R". Let F be a ray, yet more refracted, framing the blue, G yet more, causing the purple violet, these are refracted betwixt the other two to B' and P, where they paint their proper colours. But you see that it will follow they mix with the other and will cause no determinate, but a confused colour, mixed of all sorts, and (if I should urge what Mr. Newton somewhere

o The liberty has been taken of altering the letters of refer ence on the diagram, in a man

ner which was thought to make them more clear when printed.

supposes) indeed a white, which he says is the mixture of all sorts of rays. I rather think, therefore:

1. That the rays of light move not so swiftly through the prism, as in the free air, and that by several opaque particles of its substance many of them are reflected, or reverted, and at last lost in the glass:

2. That those rays, which move through a less portion of the glass prism, have their liberal motion less retarded, and consequently in their emersion move quicker, and less refracted from the way they took in the free air: so the ray HM moves swifter from the glass after its emersion at M, than either GL, FK, or EI, and retaining a greater propensity to the way it made in the free air is less refracted, and painted nearer the perpendicular than they. Whereas the ray at I, and those inceding near it, moving through the thicker parts of the prism, are many of them destroyed in their passage, and never come to emerge on that face, but those that do, having their motion much retarded by their longer journey within the glass, and a greater inclination to the refracted way impressed on them, move wider from the perpendicular, and, remotest from it, paint the blue and purple violet upon the recipient wall or scene. Hence I concluded that the red is painted by rays which move swiftly, but are refracted in no great quantities, for it is painted (in the experiment by the prism) by those rays, which proceed from the outward remotest edges of the sun; yellow is painted by rays moving not so swiftly as red, but in greater quantities; the orange is but a composition of the red and yellow on one side, as green is of yellow and blue on the other; blue is made by fewer rays than red by far, and much slower moved; the purple violet by fewer yet, and slower moved than they.

These are my first thoughts concerning colours, of which I wrote a longer letter to Mr. Wroe, an ingenuous fellow of Jesus College, Cambridge. Pray let Mr. Oldenburg see this, for I have not time to write to him, and tell him I desire his private opinion of these thoughts.

I am glad to hear that Mr. Newton's telescopes are made so well, as you intimate, by Mr. Cox. I must beg of you further to inform me, by your next, at what rates they are valued, and with what effect they perform. I mean, how I may buy one of about 2 feet, and whether it will do as well as a long one of thirty feet. As soon as I can send you money to buy me a new object glass I shall; the length of my tube is fourteen feet betwixt the glasses. Mr. Hooke procured my last, which was not very excellent. I hope he will not, at your request, refuse to try one when you speak for it, that I may not have a bad one. The two which he procured me cost thirty shillings; I shall send you forty shillings. What shall be to spare you may retain to pay for those books I owe you for, or such as I shall want hereafter. The heavens, never since I broke my glass, afforded me any opportunity for observation till yesterday, when at noon, (having covered the broken part of my glass,) with the bigger piece I took the sun's diameter 5325=31' 58", which still conspires for the exactly bisected excentricity, and the perigaon semidiameter 16' 25". I shall have occasion to write to you again ere long; wherefore, hoping only to hear again from you in the inean, at present I rest

your affectionate friend,

JOHN FLAMSTEED.

Pray impart to me what you know or hear of Mr.

Hooke's new tube. I have wrote to Mr. Nuns, but hear nothing from him. Pray inquire of his friend which way I may direct my letters, and I will write to him again.

This letter is printed in the Gen. Dict. vol. V. p. 255.

CLXXIV.

FLAMSTEED TO COLLINS.

Mr. Collins,

Derby, May 6, 1672.

Last Saturday, by our carrier, I received my father's lease; for your pains about it I shall ever acknowledge myself obliged to you. I have likewise received two letters from you, conveying the most current news, for which I am also bound to give you hearty thanks, requesting that you would continue to give me what you shall hear of certain for the future in your letters. The gazettes we have by the post, so you need not trouble yourself to send me any.

In your first letter you had something proposed by Mr. Strode for facilitating the method of finding the prosthaphæreses of the ellipsis without severally collecting the excentric and optical equations, which is done by Streete, but rather to be attributed to Dr. Ward, from whose Astronomy I believe he derived it, and the way is so facile that I cannot expect an easier. You suggest that you can find XI, XA I and AXI being given; but I cannot think that can be found without some trouble, and afterwards XA, XI, and AXI being given, it will be more difficult to find AIX than by the method of

X

A