The Knight stethoscope focused on the tension mechanism to hold the ear-pieces to the head of the physician. The design included a spring-tension mechanism which had a screw that could be turned to open or close the ear-pieces. The Charles Truax catalog of 1890 states that "Knight's stethoscope does not differ materially from the pattern of Cammann. The principal change is in the form of the spring, which in this case is spiral, acting on two levers in the form of a toggle joint." Dr. Frederick Irving Knight of Harvard credited this design to Moses G. Farmer as Knight referenced in his article in the Boston Medical Surgical Journal in 1869. The metal work is German silver, the flexible tubes covering is woven silk, the bell and junction are ebony, all characteristics of an early Knight's double stethoscope by the original maker Codman & Shurtleff, Boston. The above photos show a Knight's Stethoscope with two bells and a Flint Pleximeter and Percussion Hammer with its original wood carrying box, Codman & Shurtleff, Boston, c. 1875. The instruments are shown out of the box and inside the original wood box. The bells were interchangeable in that each could be screwed on and off the body of the stethoscope depending on the choice of the physician. The larger bell was used for auscultation of the lungs and smaller bell used to localize heart sounds. 

 

The screw mechanism became a great success and would be included in models that followed for many years.