THE once-black ink is now faded to brown, the paper a parchment-like shadow of its former self, yet the words still read as clearly as ever: "It is most certainly to you that I owe Sherlock Holmes."
If ever there were doubts that Sir Arthur Conan Doyle based his fictional detective on his former mentor, Dr Joseph Bell of Edinburgh's Royal College of Surgeons, that letter from pupil to master, certainly dispels them.
A glance at a portrait of the man who became the prestigious college's president further drives the point home: that sharp nose, the high forehead... and when it's revealed he wore a deerstalker hat and cloak when out birdwatching, he could be Holmes in the flesh.
Indeed it seems that while Conan Doyle's rich imagination created wonderfully intriguing tales which have stood the test of time, much of what he wrote was drawn from life - and in particular the people and places he knew from his time in Edinburgh.
That is what a new exhibition at the college in Surgeon's Hall - which opens next week - is aiming to prove, not least with the letter, which has been loaned to the college by the Stisted family, who are descendents of Bell.
"Edinburgh itself, the people and their names appear time and again in his books," says the college's director of heritage Dawn Kemp. "The letter is proof positive Bell was his inspiration for Holmes in terms of the deductive aspects, the analytical way he goes about getting to the root of a crime."
Indeed it goes on to state: "Though in the stories I have the advantage of being able to place him [Holmes] in all sorts of dramatic positions, I do not think that his analytical work is in the least an exaggeration of some effects which I have seen you produce in the outpatient ward.
"Round the centre of deduction and inference and observation which I have heard you inculcate I have tried to build up a man who pushed the thing as far as it would go, further occasionally."
Conan Doyle got to know Bell's analytical ways firsthand when he was his clerk at the college in 1879. He had started his medical training two years earlier and was chosen by Bell to be his assistant - in the same way Bell had been chosen by Professor James Syme before him.
It was his job to take patient notes before they were even seen by Bell - and watch openmouthed as the doctor deduced all sorts of information without even asking the patient or Conan Doyle a single question.
For instance, Bell would observe the way a patient moved - and could deduce if they were a sailor from the rolling gait. If a person had tattoos he would know where their travels had taken them. He could identify different accents, and would study their hands to help him determine their occupation - all very Holmesian.
Also on display will be Conan Doyle's clinical notebook from that time, recently bought by the college at auction for 11,000. And it seems that while he took notes, Conan Doyle's imagination was already working overtime.
"The notebook contains a draft of The American's Story which was published the following Christmas in the London Gazette," says Kemp. "It also has a poem to the Royal College of Surgeons in it which firmly places Conan Doyle in the exam room as it mentions specific specimen items which are still there in jars."
Not that Bell and Conan Doyle were ever friends, it seems. Part of the exhibition also includes Bell's journals from the 1860s until he died in 1911, and Kemp says there isn't a mention of Conan Doyle in any of them. But according to Kemp, the author also drew on other people he knew from Edinburgh for his fictional work.
"Dr Watson for instance was a friend of Bell's, a Dr Patrick Heron Watson, who ironically Bell didn't like at first but he seemed to overcome that and they would dine together often. He was a medical doctor and had a very similar background to Conan Doyle's Watson, although he was in the Crimean war rather than the Afghan.
"Then there is Sir Robert Christison, who was the professor of medical jurisprudence and an expert in poisons. His knowledge on poisons was transferred to Holmes, while in the very first Holmes' book A Study in Scarlet, there is an experiment Christison carried out in Edinburgh which is also attributed to Holmes."
That experiment was part of Christison's involvement with the infamous Burke and Hare trial. In 1829 he was called upon to examine a body recovered from Professor Robert Knox - the surgeon who bought bodies from the murderers to teach his students - to determine how it had died. Part of his examination involved beating up cadavers to prove that bruising was possible after death and were not indicative of a violent end.
"The exact same experiment is attributed to Holmes by someone telling Watson all about him, before he has even met him," says Kemp. "It's the opening to A Study in Scarlet, so proves that Conan Doyle's medical training influenced all his work, and that he soaked up a lot of information from his life in Edinburgh and then regurgitated it in some form or other in his writings.
"It's not just in the Holmes stories though. He does the same in The Lost World, as its main character Professor Challenger is based on Christison, as well as the college's Professor William Rutherford. Also when he wrote the first book he hadn't lived in London, so this idea of it being a place where Holmes would know the people he passed on the street by name is obviously taken from Conan Doyle's life in Edinburgh where it was very much the case."
Kemp's work has also resulted in a picture painted by Conan Doyle's father Charles Doyle being loaned to the college for the exhibition by its owner - which will be the first time it will have been seen in public. Called "The Bells of St Giles" it shows souls spiralling upwards from the church to heaven.
"It really is a wonderful work, especially as it is of an Edinburgh landmark," says Kemp. "We also discovered that although the first drawings of Holmes were by DH Friston, the second which actually appeared in the first book were by his father Charles Doyle.
"So yet another part of Holmes was created here in Edinburgh."
Conan Doyle and Joseph Bell, the Real Sherlock Holmes, opens on July 5 at the Royal College of Surgeons Museum, Surgeon's Hall, Nicolson Street
BACKGROUND NOTES ON GREAT EDINBURGH WRITER
ARTHUR IGNATIUS CONAN DOYLE, below, was born in Picardy Place, Edinburgh on May 22, 1859. He was the second of ten children and oldest son of Charles Altamont Doyle, an assistant surveyor in the Scottish Office of Works and Mary Foley Doyle.
His father supplemented his income by painting and doing illustrations, working as a sketch artist in court. Charles Doyle was eventually institutionalised, having become a heavy drinker and developing epilepsy.
It was Arthur's mother who encouraged his interest in literature, but his father's creativity which gave him his imagination.
After Picardy Place, the family moved to Portobello, before settling in Liberton Bank House when Arthur was five.
Educated at Jesuit schools, Conan Doyle went on to study medicine under Dr Joseph Bell at the University of Edinburgh Medical School in 1877. Bell's skill for observation made him the role model for Conan Doyle's Sherlock Holmes character. Conan Doyle went on to write 61 fiction and 48 non-fiction books.
On June 24, 1991, Edinburgh acquired a statue of Holmes in Conan Doyle's honour which stands to this day in Picardy Place, opposite a pub named after him.