i873-l Colour's and their Relations. 81 



With organic bodies it is otherwise. Among these, the 

 cellular structure more or less modified is so prevalent, that 

 it is not surprising to find that their colouring-matter tends 

 to accumulate in cells, which are easily distinguishable under 

 the microscope. These are termed pigment cells. Even in 

 cases where to the naked eye the tint appears uninterruptedly 

 continuous and uniform, the microscope shows this apparent 

 uniformity to be due almost entirely to the minuteness of 

 the pigment cells and their close aggregation. Nothing can 

 appear to the naked eye more uniform than the beautiful 

 crimson tint of certain portions of the petal of the pelar- 

 gonium, yet under the microscope the colour is seen to be 

 accumulated in curiously- formed pigment cells. So, also, 

 the skin of the negro, which to the naked eye appears of 

 a uniform very dark brown, is seen when examined by the 

 microscope to have its brown pigment accumulated in cells 

 — some large and of a crescent shape, others much smaller 

 and round. Another beautiful example is furnished by the 

 minute sea- weed Polysiphonia vestigiata, which appears of a 

 uniform red tint. Under the microscope the red pigment is 

 seen to be accumulated in cells of an elongated form arranged 

 in successive stages, a peculiarity from which the plant de- 

 rives its name. 



It has been mentioned that in the case of gold-leaf, the 

 light transmitted through the film has a different colour from 

 that which comes from its surface. This phenomenon, termed 

 "dichroism," is exhibited by several other substances — silver- 

 leaf, for instance, transmitting a blue light, while that pro- 

 ceeding from its surface is nearly white. The mineral termed 

 dichroite or iolite, a prismatic quartz, is another example, 

 its colour being deep blue when viewed in the direction of 

 the axis of the crystal, and yellowish grey in the transverse 

 direction. Crystals of augite, again, are blood-red in one 

 direction and bright green in another. The alcoholic 

 solution of chlorophyl, or leaf green, tinges the light passing 

 through it of a deep red, while the superficial colour is 

 green. In tincture of litmus the transmitted colour is also 

 red, but the superficial is blue. The change from green to 

 red in the instance of gold-leaf shows that, in some cases, 

 these transmitted tints depend simply on the state of 

 aggregation of the constituent ultimates. But in the tinctures 

 of chlorophyl and litmus, the transmitted red is due to one 

 of the constituents of those chemical compounds. 



An interesting case is presented by the tincture of the 

 bark of horse-chesnut, for it is one of transition. While 

 dichroism maybe regarded as intermediate between ordinary 



vol. in. (n.s.) m 



