One of the micrometers mounted on microscope inv. 3202 can be fitted to the guide of a brass frame. To the micrometer was fixed a second vertical frame bearing a small glass plate covered with powder obtained from butterfly wings. A brass plate with a small hole was set up in parallel with the glass plate. The lens to be examined was placed in front of the small hole. Through the lens, the observer could see the powder deposited on the glass plate. The plate's position could be adjusted by means of the micrometer, until a perfect focus was obtained. The focal length of the lens was then read on the micrometric scale.