Dear M^r Talbot,

I have just learned from Prof^r Tait, ^<1> that you have some specimens of divisions upon Glass such as used to be made by Sir John Barton. ^<2>

I have several of them in the Country; but I have tried to procure them without success.

If you could lend me your specimens for a few days you would oblige me greatly, as I am desirous of repeating some experiments which I made 40 years ago, and which I propose to communicate to the Royal Society. ^<3>

I am particularly anxious for those in which there are 500 divisions in the Inch.

I am, Dear M^r Talbot, Ever Most Truly y^rs
D Brewster

24 Charlotte Square ^<4>

Feb^y 26^th 1864

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Notes:

1. Prof Peter Guthrie Tait (1831–1901), Scottish mathematician.

2. Sir John Barton (1771–1834), Deputy Comptroller of the Royal Mint, devised a precision ruling machine that could engrave up to 10,000 lines per inch, creating a diffraction grating that produced a rainbow of colours. See P. Grodzinski, ‘A Ruling Engine used by Sir John Barton – and its Products’, Industrial Diamond Review, v. 8, February 1948, pp. 37–42.

3. Royal Society of London.

4. The largest and grandest square in Edinburgh, at the western end of Princes Street.