IN the ultra-microspectrophotometer for high resolution the absorption spectra of biological objects can be determined by utilizing the entire resolving power of the microscope. In order quantitatively to determine the absorbing substances in an object from this absorption curve the object and the measuring instruments must meet a series of conditions. For a more detailed treatment of these factors see Caspersson (1, 2). One of the factors of the greatest importance for the practical work is the requirement that the measured loss of light should be conditional only on the real absorption in the specimen. For this reason the optical system, in the first place the aperture of the condenser and of the objective, must meet certain special conditions. If the objective aperture is too small or the condenser aperture too large there will be a loss of light in the optical system, and thus part of the light leaving the object will not take part in forming the image. In the practical work on the variety of biological preparations this factor is of such dominating importance for all microspectrographic work that it must be made a condition for every curve analysis that no errors should appear owing to this factor. The most adequate method for achieving this is to determine the light distribution around the illuminated specimen, and from this distribution curve to calculate the minimum value of the permissible objective aperture and the corresponding maximum value of the condenser aperture. At this Institute a simple apparatus, constructed by L. Larsson, has been used for establishing such light distribution curves in the visible spectral range for certain critical angles. This technique is sufficiently accurate for many types of biological objects (the apparatus is described in (3)). In this apparatus the object and the illuminating system were rotated around an axis that was perpendicular to the optical axis and went through the object. The light distribution curves were determined by means of a stationary photocell. The following is the description of an apparatus which is used for more accurate measurements in regions of different wave lengths, in wide intensity intervals, and in an extensive angle-range. It is partly intended for determining the objective and condenser aperture necessary for objects in general, partly for experiments with different types of model substances. It is further equipped for making such exact measurements of light distribution that these can, if required, be used for calculating correction factors for spectrophotometric objects refracting light so intensely that the differences in the refractive index in these objects cannot be eliminated in the usual way, in which case there would be a certain error in the measurement of the light absorption itself.