This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The difficulty and discomfort amateurs experience in learning to use a microscope with both eyes open, or in trying to keep one eye shut, can be easily overcome by attaching a piece of cardboard, similar in shape to the one shown in the sketch, to the barrel Of the microscope. The hole A should be of sufficient diameter to allow the on the barrel to the proper up and down on the barrel to the proper adjustment.
This simple arrangement will relieve a great deal of the eye strain and will be of assistance to the most experienced users of microscopes. - Contributed by G. B. Fenton, Charleston W Virginia.
Ill: Shield to Cover the Eye That is Not Used When Looking into a Microscope