Faraday to Juliet Pollock   16 December 1858

[Royal Institution embossed letterhead] | Albemarle St. W. | 16 Decr. 1858

My dear Mrs. Pollock

I send you a top in return for your instruction in nine mens holes. Put the peg into the top conical end downwards about ½ an inch through & take off the little weight of tin from beneath. The top will spin pretty well though not quite true:- never mind that. Now give the handle a push so as to incline it a little towards A and spin it[.] You will have red to the left & green towards the right - or push the handle towards B & spin it - then you will have red to the right & green to the left.

Or again let the pin be upright so as to give little or no colour then stick the tin weight on beneath A spin it - you will have green to the left - red to the right - remove the tin weight to beneath B & you immediately have red to the left & green to the right.

Now for the reason[.] When the top of the stem leans towards A then the stem describes a circle as the top revolves & as the stem will lean towards A in all positions of the top so it is only when A & B are in a line with the observer that the stem will eclipse truly the diametral line. When the top is in that position in which all the green is from the observer the portion of green eclipsed will be a column leaning to the left, & when the red is all from the observer the part eclipsed will be a portion of red leaning towards the right - the two do not overlap but the green shadings are all on one side & the red on the other. So you never see the green on the left on the coloured green part beyond the top of the stem but you see the red there:- and you never see the red on the right but you see the green there & that is what you do see. If you push the stem over towards B the change of colours of course takes place. The same effect happens when any part of the green or red is shaded from the eye by the rising stem but it is most observable when the line A B is to the right & left becomes then the stem inclination is greatest in that position.

When the weight is put on the whole centre of gravity is removed to one side and the red & green shadows are more separated & stand parallel to each other but the cause is the same[.] As both coloured surface & axis move together whenever the colour has returned to a particular place the stem has returned to the same place & therefore hides the same place from the eye.

Ever Most Truly Yours | M. Faraday