Longevity of Plant Cells in Tissue Cultures.

slides over the barrel of the microscope and holds the cell flat against the ocular lens or the end of the lens barrel if the ocular lens is not used. All the light passing through the optical system of the microscope is intercepted by the cell. The camera speed is adjusted to fit the light intensity, or the light intensity is regulated by rheostat or diaphragm to conform with a desired film velocity. The cell is then removed and the camera lowered into place and the apparatus is ready for use. The unit is subject to the same limitations as the usual exposure meter. The deflection represents the average of all the light intensities present and is satisfactory when calibrated for the kind of work in question by photographic test. The use of a photoelectric device for the measurement of light values in photography is well established. The principles have been adapted to einephotomicrography in the unit above described. The total cost of such a unit is less than $30.00 and soon pays for itself in the saving of film and time.