METACHROMASY 249 



colour is said to be 'lowered', and the mean wave-length of 

 the transmitted light has indeed been lowered. The influence 

 we have brought to bear is therefore said to be 'bathochrome'. If 

 on the contrary w^e somehow shift the peak of the absorption 

 curve of our purple dye towards the left, we 'heighten' the colour 

 through magenta to red, orange, and then yellow. The influence 

 has been 'hypsochrome', since the mean w^ave-length of the trans- 

 mitted light is now higher (longer). It is obvious that if either a 

 bathochrome or a hypsochrome influence were pushed far enough, 

 so that the absorption curve were moved right outside the visible 

 spectrum, the transmitted hght would brighten to white, and that 

 is indeed why the two ends of the scale (green and yellow) are 

 rather similar. 



We are now in a position to lay down a general law about meta- 

 chromasy. The metachromatic effect is hypsochrome. This law is sub- 

 ject only to the exceptions mentioned on pp. 258 and 259. Colours 

 may be arranged in an order of increasing hypsochrome effect, 

 thus: green, blue, violet, purple, magenta, red, orange, yellow. The 

 metachromatic colour given by a dye is to the right of the ortho- 

 chromatic in this list. It follows that no metachromatic dye can be 

 yellow, for this would render chromotropes colourless. All the most 

 valuable metachromatic dyes are blue or violet, and the colour- 

 shift is generally from blue to purple or magenta, or from violet to 

 red. The normal human eye is very sensitive to these changes. If a 

 red dye had its absorption-maximum shifted by the same amount 

 in wavelength as one of these dyes, the change in colour would 

 not appear to us so striking. We seldom choose a red dye, such as 

 neutral red or safranine (azines), when we want metachromatic 

 effects. 



In passing from the orthochromatic to the metachromatic 

 colour the peak of the absorption curve may move a particularly 

 long distance; or, alternatively, the colour-shift may be one that 

 is particularly evident to the human eye, even though the peak of 

 the curve may not have moved very far. Thus a dye may be con- 

 sidered highly metachromatic for one of two alternative reasons, 

 which may be called respectively objective and subjective. 



The colour-shift could be pictured mentally as a bodily move- 

 ment of the absorption curve across the spectrum, without any 

 change in its form; but in fact the shift is more complex than 

 this.^*^' ^*^' *^^' *^^ 



