Question: Robert Armstrong-Jones the Welsh physician was what relation to Antony Armstrong-Jones, 1st Earl of Snowdon?

Context paragraphs:
Lady Margarita Armstrong-Jones: Lady Margarita Elizabeth Rose Alleyne Armstrong-Jones (born 14 May 2002) is the only daughter and youngest child of the Earl and Countess of Snowdon.  She is the only granddaughter of Princess Margaret and the first of three granddaughters of the 1st Earl of Snowdon.

Earl of Snowdon: Earl of Snowdon is a title in the Peerage of the United Kingdom.  It was created in 1961, together with the subsidiary title Viscount Linley, of Nymans in the County of Sussex, by Queen Elizabeth II for her then brother-in-law, Antony Armstrong-Jones, who married Princess Margaret in 1960.

John Donald (jewellery designer): John Donald is a British jeweller - designer whose work is strongly identified in the 1960s and 1970s in London.  Princess Margaret and the Queen Mother purchased works by John Donald in the 1960s, having been introduced to him by Antony Armstrong-Jones, 1st Earl of Snowdon.

Charles Armstrong-Jones, Viscount Linley: Charles Patrick Inigo Armstrong-Jones, Viscount Linley (born 1 July 1999) is the only son of David Armstrong-Jones, 2nd Earl of Snowdon, and Serena Armstrong-Jones, Countess of Snowdon.  He is a great-grandson of King George VI.  He is heir apparent to the Earldom of Snowdon and 19th in line of succession to the British throne, after his father.

Lady Sarah Chatto: Lady Sarah Frances Elizabeth Chatto (née Armstrong-Jones; born 1 May 1964) is the only daughter of Princess Margaret, Countess of Snowdon, and the 1st Earl of Snowdon.  She is a niece of Queen Elizabeth II and the youngest grandchild of King George VI and Queen Elizabeth The Queen Mother.

Robert Armstrong-Jones: Sir Robert Armstrong-Jones, CBE (born Robert Jones; 2 December 1857 – 31 January 1943) was a Welsh physician and psychiatrist.  He was the grandfather of Antony Armstrong-Jones, 1st Earl of Snowdon (d. 2017).

Ronald Armstrong-Jones: Major Ronald Owen Lloyd Armstrong-Jones MBE QC (18 May 1899 – 27 January 1966) was a British barrister and soldier and the father of Antony Armstrong-Jones, 1st Earl of Snowdon.

Antony Armstrong-Jones, 1st Earl of Snowdon: Antony Charles Robert Armstrong-Jones, 1st Earl of Snowdon, (7 March 1930 – 13 January 2017), commonly known as Lord Snowdon, was a British photographer and film maker.  He was married to Princess Margaret, younger daughter of King George VI and the sister of Queen Elizabeth II.

David Armstrong-Jones, 2nd Earl of Snowdon: David Albert Charles Armstrong-Jones, 2nd Earl of Snowdon (born 3 November 1961), styled as Viscount Linley until 2017 and known professionally as David Linley, is an English furniture maker and the former chairman of the auction house Christie's UK.  The son of Princess Margaret, Countess of Snowdon, and of Antony Armstrong-Jones, 1st Earl of Snowdon, he is a grandson of King George VI and is 18th in line of succession to the British throne, the first in line who is not a descendant of Queen Elizabeth II.

Anne Parsons, Countess of Rosse: Anne Parsons, Countess of Rosse ("née" Messel, previously Armstrong-Jones; 8 February 1902 – 3 July 1992), was a society hostess and one of the founders of The Victorian Society and the mother of Antony Armstrong-Jones, 1st Earl of Snowdon.

Supporting paragraphs: Robert Armstrong-Jones Antony Armstrong-Jones, 1st Earl of Snowdon
Answer: the grandfather