No longer in operation, the lighthouse was constructed in 1680 by Captains Hugh Till and Symon Bayly, with help from Thomas Ekins. Its white-painted conical stone tower is 23m tall. There are adjoining two-storey keepers’ houses, also painted white. The lamp was provided originally by a coal fire without a lens, burning in a large open basket called a chauffer, which is now on display in Tresco Gardens. Approximately a century later the coal fire was replaced by oil lamps of different designs. See Mr Abraham Leggatt's letter below this text, by Robert Heath.
From the Isles of Scilly by Robert Heath. Written in c. 1750
"About a mile southwest of the south part of St Mary's garrison lies St Agnes Island, otherwise called the Light-House Island, upon which stands a very high and strong Light-House, seen in the night at great distance, by which ships, going out or coming into the two channels avoid falling in with the rocks, lying thicker about this island, than any other of the Scilly Islands. It is also of use to all coasting vessels crossing the channels. There is nothing particular in the soil of this island different from the rest of the islands, (being, in that respect, very much alike) nor of the dwellings, or description of places, except the Light-Keeper's Habitation and employment, a Church, in use for devotion, and such like.
The Light-keeper has a salary allowed him by Trinity-Board of forty pounds a year, and twenty pounds a year allowed to his assistant, which whole sum, till the coming of this last Light-Keeper, (Capt---Clark) used to be allowed to one person, without any assistant. He is also allowed a dwelling-house, and a piece of ground for a garden by the Trinity-Board, as has been formerly the custom. And considering his close confinement upon this remote island, and the care required here to keep a good light, more than in others places, upon the English coasts, encouragement ought to be given to a Light-Keeper, where our navigation, and the lives of his Majesty's subjects, are the most depending, by an augmentation, rather than a reduction of his salary. This light is kept with coals burning near the top of the light-house, which being laid on in large quantities, and sometimes stirred with an iron rod, the ruddy heat and flame are strongly perceived, thro' the glass frames, surrounding it, at a vast distance upon the sea; yet before the coming of this present Light-Keeper , I've known it scarcely perceivable in the night, at the island of St Mary, where it now looks like a Comet. And some are of the opinion, (not without reason) that in the time of the former Light-Keeper it has been suffered to go out, or sometimes not lighted.
It is supplied with coals by an annual ship which comes freighted on purpose. The hire of the carriage of which the coals to the Light-House, from the sea side, where they are taken out, is an agreeable benefit to the poor inhabitants. The top of the Light-House, ( from whence, in the day time, I have I have taken a view, fifty or sixty feet from the high ground which it stands upon) commands a very wide, and remote prospect, upon the neighbouring seas.
What is further remarkable concerning this island, is that by its situation, next to the numerous Western Rocks, more wrecks of ships are sent in here by the sea, than to any other of the Scilly Islands; which make the inhabitants of some amends for their forlornness of abode. St Warna, (by some called Santa Wauna) whom these people invoke, as their benefactor, in time of distress, is supposed to be instrumental in sending these wrecks, and of directing and presiding over their good fortune.
About the middle of this island is a cavity of small depth, sunk in the earth, consecrated to the memory of this saint, or Holy Spirit; in honour and gratitude to whom several of the inhabitants pay their annual devotions at the place, on the day after Twelfth-Day, cleaning it out, and using certain superstitious ceremonies in their thanksgiving; which being ended, they make a general feasting and rejoicing throughout the Island.
Letter written by Abraham Leggatt to the Political Magazine in September 1783:
"Caution to Merchants, etc. who sail to the westward of the Scilly Islands."
"I desire you will insert for the public good, the following caution to merchants in general, and the captains and masters of vessels in particular, who sail to the westward of the Scilly Islands. And that it may bear the mark of authenticity, you may make use of my name. Signed Abraham Leggatt."
Letter: "The Light-house at Scilly heretofore has been held in the highest estimation, and consequently was equal, if not superior, to the best light in the kingdom; frequently distinguished six or seven leagues. The alterations of contracting the lanthorn, and burning oil instead of Kennel coals, has undisputedly placed it amongst the very worst, not withstanding the great assiduity of the light keepers. In the clearest night in wears only the appearance of a small star; when hazy, or blowing fresh, like unto a small candle in a common lanthorn, and at a very small distance, not to be distinguished from a ship's light. Men who fish during the night round the small island on which it is placed, at times cannot see it at all. Vessels of late, during the night, have frequently fell in amongst the rocks, and but for the fishing boats, must have become wrecks.
I am sorry to add, the instances that happened on the 4th September at night are too melancholy proofs of what is advanced! At ten o'clock, struck on a rock, a boat two miles from the Light-house, the Financier (formerly the Lord Howe transport), from Charlestown, South Carolina with tobacco, rice and indigo etc. bound for London, John Lobec, master. She went to pieces in 15 minutes, the mate, one man and a black servant lost, the rest of the crew, with a gentleman, his lady and another passenger were saved on a raft of the wreck, and came ashore on a small island near; some of them much wounded, nothing saved.
About the same hour struck the ship Nancy, David Robertson, master, from Jamaica to London, with 203 puncheons of rum, a large quantity of sugars, and ?; the crew all saved, vessel lost, and nearly all the cargo. This ship lay on the rocks within a mile of the Light-house.
A large snow, observed by one of these ships, is supposed to have gone down near the same spot, and every soul perished, as a head and quarter boards are cast on shore, which prove not to be part of either of the other ships. This vessel set Hamburgh colours.
Nearly at the same hour a brig Henry, Mr Thomas Daly, master from Jamaica to London, with rum and sugar, had a most miraculous escape from destruction. No pilot in open day, with a fair wind, would have attempted to carry him by such rocks, and through such a passage as he must have passed before the vessel ran on shore; which providentially was on a bed of sand; with great help, much expense, and some damage, this vessel was got off, and is ready to proceed for London; no lives lost.
On Monday, the 8th at daylight, a snow from Lisbon, bound for London, was amongst the rocks, but the wind being fair to easterly, her in a one found and at another which the master had some knowledge of she sustained no damage.
Query: Is it not a bad light that deceives the mariner worst than none at all? Be that as it may, I hold it a debt due to humanity to lay these plain truths before the public, for without the greatest care to avoid and interposition of the Almighty to defend, many vessel sit is believed, will be lost on the rocks of Scilly in the course of the winter.
St Mary's, one on the Scilly Islands, Sept. 10, 1783. 
It is possible that improvements were made by 1790 viz:
Times: Thursday, November 25, 1790, Issue 1736 - St Agnes, LIGHT-HOUSE.
“The improvement lately made in the Light upon St
Agnes, one of the islands of Scilly, is spoken of very highly by nautical men
who have seen it, as very well calculated to answer the end proposed- namely, to
give a stronger light, and so to be distinguished from any other light in the
Channel. The light makes a revolution once in a minute, and consequently shows
itself like a brilliant star, or flash of lightning, in every direction that
cannot be mistaken, and requires only to be generally known to be as beneficial
as it is intended.
The important situation of Scilly, at the very entrance of the Channel, surrounded with numberless rocks, makes this light of the utmost consequence to seamen, whose lives, as well as immense property, often depend upon its being seen. Too much praise therefore cannot be given to the Corporation of the Trinity House for their attention to, and present improvement of it.”
Murder Mystery - Research by Trevor Newman.
So the story goes, that
in 1809 the two keepers (not Scillonians) didn't get on, at all, and all the
island knew of the increasing vehemence of their arguments. One day one of
the keepers was nowhere to be found. He was never seen again, and it was
presumed that he had committed suicide. His fellow keeper was removed from his
post shortly afterwards, and presumably left Scilly. Then in 1848, when
the present-day adjoining lighthouse buildings were being built, as the workmen
were preparing the foundations they found a skeleton buried only three feet
beneath the surface. Some of the islanders recognised the skeleton as that of
the keeper who had gone missing in 1809, by peculiarities in his teeth. Hence it
seems that he had been murdered by his fellow keeper, and buried in a shallow
grave near the lighthouse.
Trinity House Records
Death of Adam Walker
Times: Tuesday, February 13, 1821, Issue 11169 – St. Agnes; LIGHTHOUSE
Deaths –“ On Sunday last, at Richmond, aged 90, Mr Adam Walker, the late celebrated Lecturer in Experimental Philosophy. His ingenious mind was active in the pursuit of science, and his original invention of that beautiful machine the Eidouranion, or transparent Orrery, and the Celestina, the great revolving lights on the Isle of Scilly, and Cromer, by which (under Providence) thousands of lives and property have been saved, the warm air stove under the House of Lords and Italian Opera-house, the present mail-coach, &c, still remain as proofs.”
Notes & References:
 -This letter is quite interesting, because written in 1783, it would suggest that this is when the oil lamp was fitted rather than 1790 as often stated. Unless of coarse he is referring to an earlier inferior lamp. 1790 probably refers to the fitting of the rotary light mechanism by inventor Adam Walker.
2 - A must read:
'Neglectful or Worse' - A Lurid Tale
of a Lighthouse Keeper and Wrecking in the Isles of Scilly by Cathryn Pearce
- The Online Journal of the National Maritime Museum Cornwall. 2008, Volume 1,
3 - Chaplin, Capt W., ‘Story of St Agnes Lighthouse in the Isles of Scilly,’ unpublished manuscript, Corporation of Trinity House Library, No Date.