A high speed recording microdensitometer suitable for cytological research.

SEVERAL methods in biological and biophysical research involve recording quantitative information photographically, and for these purposes the accurate and speedy measurement of the density of film or plate is therefore becoming increasingly important. Most commercial microphotometers have been designed to measure spectrographic plates, recording transmissions substantially along the major axis of the plate by means either of a galvanometer and bromide paper or a commercial potentiometer recorder. Transmission measuring instruments are unsuitable for many applications such as absorption measurements of biological cells, where it is often necessary to integrate the density over an area, and inconvenient for many others. Further, the line scanned may have to be placed at any angle across the specimen and exactly related to the often complicated photographic image. Since optical attenuators are available which obey a density law, it is possible to make a true null-point instrument which will record densities automatically by pen on paper, without the need for an external recorder, if advantage is taken of modern servo-motor and amplifier design. Direct pen recording also confers the secondary advantage that specimen and recording paper can be directly linked together, which makes it much easier to locate and compare detail on trace and film. We have already made and described such a recording microdensitometer [2], which was designed in particular for measuring the densities of the small negatives exposed in the ultra-violet microscope. Owing to the development of other methods of investigation involving various sizes of negatives it was decided to build a more universal microdensitometer which might also act as a prototype for a commercially available instrument, which is required by many workers in the field of microspectrography [3], cell interferometry [l] and X-ray diffraction 141. The principle of operation is based on the well-known double-beam system in which the two light beams traversing different paths are made to fall alternately on to a photo-sensitive detector. If the intensity of the two beams is different an AC. signal is produced by the detector which, after amplification, causes a motor so to move an optical attenuator as to reduce the intensity difference. In this way a continuously self-balancing system is obtained in which the position of a suitable attenuator can be made to record the optical density of the specimen. It is not the purpose of this communication to describe the microdensitometer in detail but to indicate its general features and performance, which may be useful to cytologists and others requiring to measure film densities from detailed photographs.