Marking the origin of coordinates in high-resolution light-microscopy scanning

In high-resolution image analysis, it is often desirable to return to a chosen cell after it has been restained or subjected to histochemical procedures. The reading of the vernier on the microscope stage is too coarse for relocating of non-distinct single cells, because the accuracy, determined by visual interpolation, is limited, at best, to 1/20th of a millimetre, or 50 microns. When one works with haematologic samples, e.g. lymphocytes, the precision of relocating a cell has to be better than 50 microns; i.e. the cell should reappear near the centre of the visual field of a X 100 oil-immersion objective. We describe a simple device by means of which distinct marks can be made on a slide with specimen (but before coverslipping) and will provide suitable origins for a coordinate system that will cover the entire preparation.