When you set out to write a historical novel, whether that is set in prehistory, medieval England or the American Old West the first thing you have to do is immerse yourself in that time period. One thing you can be sure of is that if you get the historical details wrong then some of your readers will probably stop reading at that point in the story without enjoying the rest of the adventure. You have to pass the test of time.
We are talking about anachronisms. The word comes from the Greek ana, meaning 'against' and chronos, meaning 'time'. Effectively it refers to an inconsistency in time. An obvious example would be having a telephone back in the days of King Henry VIII, or having a character in the 19th century using a slang term from the 21st century. It is acceptable to have these things occur in a Sci Fi time story, or even in an alternate universe or Steampunk novel, wherein you have created an alternative time and technology, but it is not acceptable in historical novels.
The Wimshurst machine, an electrostatic generator was invented between 1880 and 1883
Yet it is easy to slip them in by accident if you just assume that certain things had been invented 'round about' a particular time. The only way to guard against it is to be meticulous in your research, getting things pinned down to exact dates.
As a doctor I introduce medical details into most of my novels and stories. I make sure, however, that I have researched the minutia about the medical and surgical instruments that I have my characters use and I make sure that they are practising exactly as an actual doctor of the time would practice. That means I go back to primary sources. I consult medical textbooks of the time to ensure that particular illnesses were known about then, that surgical operations or techniques had been developed and that a treatment outcome would be plausible. While books and films often have someone half-dead or in the advanced stages of septicaemia brought to a town doctor in the 1870s who almost miraculously cures them because he is so skilled, the reality would probably be quite different. Don't get me wrong, my characters do perform cutting edge (for the time) operations but I make sure that what they do would be feasible, using the techniques and facilities of the time.
Let me give you an example. Don't have a doctor in the Old West taking somebody's blood pressure. It was not understood. Indeed it was not until 1896 that Scipione Riva-Rocci invented the sphygmomanometer. This was done using a mercury manometer. It was subsequently refined by Von Recklinghausen in 1906, which used a moving needle to measure the pressure. In neither of these was a stethoscope used. Indeed, the stethoscope was still a relatively new instrument and it did not occur to anyone that there were useful sounds that could tell one about blood pressure.
Having this early 20th century sphygmomanometer to measure blood pressure in the Old West would be an anachronism
A Russian doctor, Nicolai Korotkoff (1874-1918) wrote a thesis about his use of the stethoscope to measure blood pressure in 1910. It was entitled Experiments for determining the strength of the arterial collaterals. In this book he outlined the method of taking the blood pressure that has been used right up until the present day. But it wasn't used in the 19th century; that would be an anachronism.
So too it is with my character of Doc Marcus Quigley, itinerant dentist, gambler and bounty hunter, whose adventures I write for High Noon Press. He practices with the dental equipment of the time, using the techniques then known.
Dental toothkeys were going out of fashion in the late 19th century, but some dentists still used them
It is actually great fun doing the historical research when you write a western or any other type of historical novel. Steep yourself in the time period and get a feel for it. Makes copious notes and find out when certain things were invented and when they drifted into common usage. Make sure you pass the test of time.