HIPPOLYTE VARIANS. 595 



their kindness in giving us the facilities necessary for the 

 prosecution of these researches. 



Section I. 

 Methods. 



There are in any research on colour-change three essential 

 requirements: a standard illumination, observation 

 under constant light conditions, a means of rapid 

 record of the observed colour. No one of these have 

 we been able to meet satisfactorily. 



As our standard illumination we employed light from an 

 incandescent gas burner. 



Our colour records were made by observing Hippolyte 

 against the white background of a porcelain dish. 



In recording colours we at first attempted to supplement 

 verbal descriptions by the use of Maxwell's colour top, but 

 we were compelled to fall back upon water-colour sketches. 

 We now content ourselves with a terse note of the colour 

 and a careful description of the condition of the " chroma- 

 tophores," as seen by transmitted and by reflected light 

 under the microscope. 



The experience we have gained leads us to think this 

 last mode of recording may, after it has served as an aid 

 in unravelling the complexity of the " chromatophores " 

 themselves, prove alone sufficient as a colour record. 



It is necessary, of course, to note the condition of the 

 " chromatophores " at the first moment of examination, since 

 these bodies, or rather their pigments, are so susceptible to 

 light as to immediately and profoundly change their con- 

 dition when exposed to the powerful light necessary for 

 microscopic observation. When the animal under obser- 

 vation is not required for further experiment, change in its 

 colour condition may be considerably retarded by placing it 

 in water near the freezing-point, or, in some cases, by drop- 

 ping the animal into water of about 90° F. Hippolyte in 

 the latter case rapidly succumbs, but colour-changes in the 



