… nature often produces combinations and effects which on paper appear incorrect. - Alfred Horsley Hinton

Photography itself may err by inaccurately rendering the relative tones in Nature. Then we shall have to ask, What is " Tone "?...

The louder a sound is, the more we recognize it as being near, so the louder the "tone" of objects that is, the blacker or whiter the nearer they seem; and so if in our picture we wish to give a sense of distance, we must see that the darkest shadows and highest lights are in the foreground : and because we may not be able to materially alter things as the undiscriminating process gives them to us, we must seek for and select those scenes, those subjects, in which this arrangement of highest and deepest tones do come in the foreground, and then take care that our process renders them with fidelity, so that we may not lose the sense of their nearness or the feeling of greater distance of other planes which it is intended they shall give.

The different degree of light interference between blue and red also affords a power which we may be glad to avail ourselves, in some cases using the red where a very marked degree of lightening is desired, or blue where only a trifling difference is needed.