154« PROFESSOR FRANZ BOLL. 



to be the case with the rods of heteropoda, and former 

 observers have already called attention to the red colour of 

 the optic rods in Crustacea, lepidoptera and coleoptera. I 

 had the opportunity at Viareggio of examining numerous 

 species of cephalopoda and massive Crustacea in the fresh 

 state, and found in all, without exception (in the cephalopoda 

 in the layer of rods, in the Crustacea in the optic rods, which 

 are composed of five plates), exactly the same purple colour 

 which I had discovered in the retina of vertebrata. In these 

 invertebrata it is much more intense, and is preserved much 

 longer and better than in vertebrata. 



It seems, therefore, to be a general rule that the peculiar 

 substance composed of superimposed layers of plates, which in 

 the retina of vertebrata forms the external segments of the rods 

 and cones, and in the eye of invertebrata physiologically 

 (perhaps also phylogenetically) equivalent organs (the rods of 

 cephalopoda, and the optic rods of arthropoda), shows a very 

 characteristic purple-red colour in all these animals, which 

 is very intense during life, and mostly fades with extreme 

 rapidity after death. 



This purple colour appears to be completely identical in 

 the eyes of all animals that have been hitherto examined. 

 My friend and colleague M. Blaserna, who observed the colour 

 of the retina of a frog with a small spectroscope while I was 

 preparing it, immediately determined its entire difference from 

 the colour of haemoglobin, and further decided that it does not 

 correspond to any simple spectral colour, but must be con- 

 sidered compound. A confirmation of the correctness of this 

 view may be derived from microscopic examination. For some- 

 times the colour of the first stage is continued long enough 

 to enable one to decide in a microscopcal preparation of the 

 frog's retina that in the mosaic of the layer of rods, when 

 focussed by the microscope, all the optical transverse sections 

 of the rods do not appear red, but that among a very large 

 majority of red circles a minority (about ten per cent, on a 

 superficial reckoning) appear of a greenish-blue tint. 



Concerning the nature of these colours of the external 

 segments of the rods, I hope soon to give further explana- 

 tions in a subsequent communication, and especially to be 

 able to decide the question whether they depend on a special 

 colour of the plates themselves or whether they are due to 

 the optical properties of the plates, these being themselves 

 colourless. In the latter case they would be classified among 

 the phenomena of interference. 



In this subsequent communication I will further develop 

 more completely certain physiological corollaries, which 



