The newly acquired microscope came with two objectives on a three-position nosepiece. One empty spot to fill - ideally with a low power objective. The 10x objective is of the divisible kind, the lower portion can be unscrewed to change from a 10x to a 4x objective. (Snippet from the December 1939 issue of Popular Science.)
Even nicer would be to fit a 4x objective at the empty position. With the amazing global flea-market that is the internet, one such objective was found to be for sale. Even though it was located in Santa Barbara it was easily purchased and shipped with amazing speed to the Low Countries.
For being nearly 90 years old by now this objective is in fair shape, only one small blemish (nick) on the front lens. Whilst this will reduce sharpness in a part of the image, it is not in focus so not too damaging. This would probably make the objective be rejected for laboratory work, but for hobby-microscopy at low powers it is still fine.
To clean the objective, used demineralised water with a soft cloth. A 'camel-hair brush' to flick off larger dust or particles. When carefully wiping clean lens surfaces of 'condensed' dirt, regular tap water would evaporate and leave a thin film deposit on the lens. With demineralised water, it can be gently (!) wiped clean and dry.
With three objectives of the right pattern fitted in the nose-piece, it does look the part again. Also it now works fine for magnifications from 20x up to 430x. For practical, hobby use, that's plenty of power.