Page:Principles of Microscope.djvu/34

 6 which have been enumerated above may be said to define, in a satisfactory manner, the configuration in plan of the object.

In connexion with the combined colour and outline picture a difficulty may arise as to whether the proper contour is that which runs along the confines of the coloured area, or that which follows the perimeter of the outline. The nature of the difficulty will be clear to us if we call up before us the situation which would arise if it were proposed that craniometrical measurements should be made upon the heads of the saints as depicted in heavily leaded stained glass windows. We might here be in doubt as to whether the leaded outlines were to be excluded or included in the measurements.

In the matter of their capacity for disclosing configuration in relief, important differences emerge as between the different types of picture. Bright outlines upon a dark background furnish only an external system of exterior outlines, and give in conformity with this a picture entirely upon one plane. We have striking illustration of this in Figs. b, Plates II and III. The same defect attaches, though in a less conspicuous manner, to the dark outline picture. On comparing Fig. a, Plate III, with Fig. c on the same Plate, it will be recognized that the configuration in relief is only very incompletely brought out by a system of outlines. A colour picture discloses in a more adequate manner the configuration in relief. To take a concrete example the difference in the depth of the colour as between the centre and the periphery of the red corpuscles, as depicted in a colour picture, would provide a satisfactory clue to the interpretation of their shape. But best of all from the point of view of the disclosure of the configuration in relief is the picture in light and shadow, or, as we have called it, the picture in relief. Referring again to Fig. c, Plate III, we see that the concave shape of the red blood corpuscles is here directly enforced upon the attention.

When we come to realize, as we may here in anticipation, that dark outline pictures and colour pictures respectively as obtained with the microscope are in each case furnished by light transmitted through the object, while the picture in relief is obtained in part by the reflection of light from the external surfaces of objects, it is borne in upon us that the two former types of pictures will be appropriate for the disclosure of the internal structure as well as the configuration in plan, and that the latter type of picture will