which is represented in Fig. 1. Here the outlines which produce the characteristic stellate cross are in reality the delimiting outlines of two four-sided pyramids which project respectively upwards and downwards from the optical plane upon which the external outlines of the figure are disposed.
Fig. 1.
AN OXALATE OF CALCIUM CRYSTAL.
The second variety of the fallacy will find illustration in Experiment 2 below, and also in Experiments 4 and 5, Chapter V sub-sec. 3.
The third variety of the fallacy finds illustration in Experiment 1 described below.
Experiment I. Turning to Plate II, Fig. e, dispose this first at a convenient distance from the eye, and then bring it nearer until its structure can no longer be seen in sharp focus. It will be observed that at the point at which accommodation gives out a process of inversion takes place, which gives us, instead of the pattern of dark discs included in white hexagons, a pattern of white discs included in dark hexagons.
Experiment 2. Take now a pocket lens before the eye and bend over the picture until the pattern comes into view in accurate focus. This done, carry the head closer to the paper until the picture is thrown out of focus. The pattern will now, as in the last experiment, be inverted. Now draw back from the paper beyond the point at which a focussed picture is obtained. The pattern of dark discs included in white hexagon will once more give place to a pattern of white discs included in dark hexagons.[1]
B. Subjective Colour Contrast. Whenever a coloured field is chequered in such a manner as to furnish areas on which the ground colour is subdued by shade, colour contrast phenomena will manifest themselves given that the patches in question radiate uncoloured light to the eye.
The practical importance of this fact in connexion with microscopic work will be recognized in a later stage. For the present it will suffice if the reader will turn to Plate II and will acquaint, or
- ↑ This figure may be employed as a convenient test for presbyopia or hypermetropia. Considered as a test for these conditions, it presents the great advantage that the critical sense of the patient will, provided that a perfectly regular geometrical figure is employed, be every whit as well satisfied by the unfocussed and "inverted" as by the duly focussed and "uninverted" image.
