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532 J. HRDUCKA [j. s. M. p. E.
first surface of the objective. On the other side, and as close as possible to the objective, was placed the sensitometer with the photographic plates.
I used a non-intermittent sensitometer of my own construction, with a rotating disk. The measurements were carried out as follows: The source being placed at the focal point (of the objective), the distance r\ from the source to the first surface of the objective is measured. If d\ is the diameter of the diaphragm and dz the diameter of the corresponding exit pupil, calculate r2, where
This is taken as a fundamental distance. The diameter d2 is determined photographically. The source being at the focus of the objective, a reading is taken with the sensitometer, after which the objective is removed and the source is brought to within a distance, A of the sensitometer, where
A = / (f2 fl) ................ (3)
where / is the total length of the objective; i. e., the distance from the front surface of the first element to the rear surface of the last element. Take the intensity given on the photographic plate by the source in this position as unity. Proceed to take other sensi tome trie readings with the source nearer and farther away than the distance A. Measure the densities produced on the various negatives in a small region containing the optical axis. Draw a curve plotting these densities against the distances from source to sensitometer (with objective removed) and determine, by simple interpolation, the position of the source (without objective) for which the illumination produced on the photographic plate is equal to the illumination given on the plate exposed behind the objective. The value of this distance, which we shall call r, enables us to calculate this illumination by the simple equation:
.7 r22*
A "IT .......... ••< ......... <4>
This experimental method has certain advantages. Use is made of a point source and of narrow sensitometric plates (4 mm.), and
*Note: For the sensitometric record made with the lens in place,
E = I/rf see (2)
For the sensitometric record made with the lens removed,
E. Io/r> But since E = Eo /.///o = •