Jt^otes oil Mi)ruipoda. 15 



lowered and the animal is secured with the two electrodes in 

 contact with its body. The current can then be a])[)lit!d l)y 

 luyinj^- contact wires from the induction-coil one upon each 

 electrode of tlie hohler and completing the circuit in a dark- 

 room by means of a switch or push. A slijijht modification 

 of tliis arrangement mnkes it possible to observe controlled 

 luminosity under the microscope (PI. I. fig. 8). 



For photographic records of luminosity we have adopted a 

 difi'erent method. In this case, in order to obviate the 

 possibility of a photographic record of stray electric sparks, 

 it is safest to use pressure alone to stimulate light-produc- 

 tion. It is necessary to carry out the experiment in a 

 photographic dark-room. The lower glass plate of the 

 oidinary holder is replaced by a piece of photographic film 

 with the sensitized surface placed downwards (outwards), so 

 that no contact action upon tiie prepared surface of the film 

 can be initiated by the secretions of the centipede's body. 

 The upper glass plate of the holder is replaced by a dull 

 metal sheet. The animal is then allowed to crawl upon the 

 back of the photographic film and upon it is laid the metal 

 j)late. Pressure upon the metal sheet stimulates luminosit}'^, 

 which is photographically recorded on the film (PL II. 

 fig. 13). The animal should be killed immediately by 

 dropi)ing it into methylated spirit. 



lu taking microphotographs of the ventral surface of 

 tliese centipedes to show such features as the pore-field 

 and the integumentary glands the holder is again employed 

 n})on the stage of the microscope, but it is well to weigh 

 evenly the uppermost plate as an additional precaution 

 a<iainst slii^ht bodv-movements. In such cases the movement 

 of walking a|)pendages may be neglected (PL I. fig. 9). 



In the estimation of the intensity of light we have not 

 attempted any very delicate measurement. In some early 

 experiments we were entirely guided by the eye, and taking 

 the initial intensity as our standard we gave it the arbitrary 

 designation 10. Later we found that a luminous powder 

 used by clockmakers when mounted in Canada balsam served 

 a useful purpose ; we estimated the initial luminosity of a 

 stimulated centipede to be about 100 times brighter, and with 

 this assumption, taking the luminosity of the test-slide of 

 clockmaker's powder as one unit of brightness, we were able 



the apparatus is often easier to manipulate when the electrodes are free, 

 and this because dilhculty is generally e.vperienced in keeping the 

 animal in its proper position luitil it is secured by gently lowering tho 

 upper plate upon it. 



