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Messrs. F. W. Keeble and F. W. Gamble. 



III. The nature of the " chromatophores " and their pigments. 



IV. The habits of Hippolyte varians. Sexual dimorphism. 

 Y. The nocturnal colour. Nocturnes. 



VI. Periodicity of colour-change. 

 VII. Range of colour-change. 

 VIII. The causes of change in colour. 



a. Colour of the surroundings. 



b. Light-intensity. 



c. Electric and other stimuli. 

 IX. The role of the eye and nervous system in the control of the 



colour-form. 

 X. The " chromatophores " of larval forms. 



I. Previous Knowledge. 



The facts previously known may be arranged in three groups- 

 The great variety of colour displayed by different specimens of Hippo- 

 lyte varians ; the " mimetic resemblances " between these colour-forms 

 and the Algae upon which they live; and the power which these 

 colour-forms possess of undergoing a change of tint under different 

 conditions of illumination. It is known that the so-called " chromato- 

 phores " contain differently coloured pigments by which both the pre- 

 valent tint of the prawn and its colour-changes are determined. The 

 arrangement of these colour-elements in the body and the pigmentary 

 conditions of the various colour-varieties have not been hitherto care- 

 fully examined. The factors which determine a change of colour, the 

 extent of these changes, and the mode in which they are effected, may 

 be said to be hitherto quite unknown. 



II. Method of Obtaining Reliable Colour-records. 



(a) Records of colour must be made under constant conditions of 

 illumination, otherwise they are not strictly comparable, and fine 

 shades of colour escape notice. They must be made rapidly, otherwise 

 the light used for recording induces a colour-change and becomes, in- 

 stead of a guide, a source of error. Finally, the light must be such as 

 to enable a speedy record to be made, and yet one which itself induces, 

 a minimum change. 



Many devices have been tried, but no completely satisfactory method 

 has been obtained. We use, as most convenient, bright diffuse sunlight 

 reflected from a white ground ; and for a comparison of day and night 

 colours, incandescent light. There are, however, several objections to 

 these modes of ill animation, the chief among them being that under 

 certain circumstances, white light produces a very rapid change of 

 colour. 



