SVK Test Model 5. This model was the same as the fourth, except that the lens was eliminated. The limitation of the field of vision of the cell was determined by the size of the window opening and the distance of the cell from the window. It was found that units consisting of four or five selenium cells with a diameter of about three inches were most suitable. (Although seven cells connected in series proved more efficient, it was felt that the greater expenditure in construction was not warranted.) Tests of this model gave results similar to those conducted with the fourth model.