(c) Objects may be defined both by outlines and by a difference of colour. The corresponding pictures may be spoken of as combined outline and colour pictures.
The outline pictures and colour pictures spoken of in (a) and (b) may be distinguished from those here in question as pure outline or pure colour pictures respectively.
(d) Objects may come into view as patches of dark and bright, the former corresponding to prominences and illuminated surfaces (understanding here by illuminated surfaces, surfaces which radiate light to the eye), the latter to hollows and shaded surfaces (i.e. surfaces which radiate no light to the eye). The corresponding pictures may, according as we are dealing with uncoloured or coloured objects, be spoken of as pictures in relief or pictures in colour and relief.
Figs, a, b, c, d and e, Plate I, furnish examples of the dark outline picture, the bright outline picture, the picture in relief, the pure colour picture, and the outline and colour picture respectively. Further examples of the first three are furnished respectively by a, b, and c, Plate III ; further examples of colour pictures by d, e, f, g, and h of Plate III.
We have familar every day examples of outline pictures in outline drawings in black on white and white on black. Ordinary artists' paintings and coloured transparencies would come under our heading of colour pictures. Leaded stained glass windows, and the daubs produced in the nursery by the superposition of colour on outline drawings, would constitute colour and outline pictures. Photographs of statuary and of the surface of the moon, lit from the side in such a manner as to give a stereoscopic effect, furnish familiar examples of pictures in relief.
3. Preliminary consideration of the respective advantages and disadvantages of the different varieties of pictures enumerated above.
A comparison of the respective advantages and disadvantages of the different types of pictures enumerated above involves in each case an inquiry into the capacity of each picture :—
(a) For rendering the object visible when it subtends only a small angle to the eye ;
(b) For disclosing its configuration in plan and relief ;
(c) For making manifest, in the case when it is transparent, its internal structure.
(d) For furnishing an image which shall not be liable to erroneous interpretation.
