30 PRINCIPLES OF MICROSCOPY
the microscope, place a drop of cedar oil or of Canada balsam upon it, and illuminate from below with coloured light.
It will be plain now even to naked eye inspection that the coloured light passes freely through the paper in the region of the oil spot.
Examine the preparation by the aid of the microscope, and observe that now the coloured light floods through every pore of the paper over the area occupied by the oil spot. (Plate II, Fig. ; on the left.)
Take note also in passing of the fact that under the influence of the colourless light which falls upon and is reflected to the eye from the more opaque strands of the paper, these stand out in beautiful colour contrast to the ground tone furnished by the transmitted light. (Plate II, Fig. j seen through the covering tissue paper.) The brilliancy of the colour contrast will, in conformity with a general law, be greatest where the illumination of the coloured and uncoloured areas of the preparation is exactly balanced.
Experiment 5. Repeat the above experiment, employing, as in Experiment 2, blue paper and a red glass in the sub-stage. Note that now the paper everywhere transmits red light to the eye. 1
Experiment 6. Take in hand again the strip of white filter paper with the oil spot. Replacing it upon the stage of the microscope, cut off the light from below, and illuminate from above with coloured light. Note that the oil spot will now appear uncoloured, while the surrounding paper will reflect coloured light to the eye. The transparency which is achieved by filling in the interstices of the preparation with a highly refracting medium is thus seen to go hand in hand with loss of reflecting power.
Experiment 7. Place side by side under the microscope a dry strip of filter paper and a strip of filter paper which is impregnated with oil. Closing down the iris diaphragm, screen off the top light and examine the fibres in the region where the interstices are occupied by oil, and again in the region where the interstices are occupied by air. Notice that the contours of the fibres in the former case are, as shown in the figure (Plate II, Figs, i and j on the left), delicately outlined, and that they are in the latter case broad and dark (Figs, i and j on the right).
The gain in transparency is thus seen to be associated with a loss in refracting power on the part of the fibres.
5. Deductions from the foregoing experiments.
We learn from these experiments that transparency is achieved only at the expense of those optical differences which are essential to the development of an object picture of an uncoloured element. The necessary corollary of this is, as we shall see, the abandonment of the outline picture and the resort to a pure colour picture in every
1 This quality of transparency which is conferred upon filter paper by mounting it in a medium of high refractive index was, as the reader will now appreciate, turned to practical account in Cap. II, Sect. 2, subsec. 2, in connexion with the making of the parti-coloured stop employed there
