It is difficult to realize how one may understand the theory of a method, and may employ it habitually in practice, and yet may attain results of insignificant value, as compared with what might easily be reached by a little more exactness. The difference between a loose method and a method of precision may be nothing in theory and trifling in detail, but enormous in practical usefulness. A scientific method becomes a method of precision, and attains practical value by comparatively slight modifications, but these may be made or adopted very slowly.
We can better understand how slowly by looking at the history of a diagnostic procedure that is now generally relied on. It is more than three hundred years since the invention of the thermometer. About two hundred years ago it was perfected in practically its present form by Fahrenheit. Yet physicians still in active practice remember the work