The marvelous advance that modern medicine has made is due in great part to the invention of instruments of precision. They have lent delicacy and accuracy to our methods of examination and have opened new fields that were closed to our unaided senses.
When early in the last century Laennec invented the stethoscope, he gave us the means of discovering morbid processes within the body and thus revolutionized internal medicine. Helmholtz's invention of the ophthalmoscope did the same for ophthalmology. In both cases the field had long been cleared and there was no reason why the instrument should not have been invented earlier, but that in the one case it required the genius of Laennec and in the other the versatile and profoundly scientific talent of Helmholtz to lay bare simple facts which every tyro saw plainly after they had once been pointed out.
As the invention of the stethoscope
FRIEDENWALD H. THE HISTORY OF THE INVENTION AND OF THE DEVELOPMENT OF THE OPHTHALMOSCOPE.. JAMA. 1902;XXXVIII(9):549-552. doi:10.1001/jama.1902.62480090001001