BRIGHTNESS, BUT NOT COLOR. 221 



that they perceive all the luminous rays, and can dis- 

 tinguish very slight differences of intensity ; but that 

 they do not distinguish between different colors. He 

 sums up his observations as follows : — 



" II resulte de ces experiences que ce qui agit sur les 

 Crustaces, ce n'est point la qualite de la lumiere, c'e-t 

 exclusivement sa quantite. Antrement dit, les Crus- 

 taces infeiieurs ont la perception de toiite onde lumi- 

 neuse et de toutes les differences, meme trbs legeres, dans 

 son intensite; mais ils ne sont point capables de dis- 

 tinguer la nature des ondes, de differentes couleurs. lis 

 distinguent tres bien I'intensite des vibrations etherees, 

 leur amplitude, mais point leur nombre. H y a done, 

 dans le mode de perception de la lumiere, une grande 

 difference eutre les Crustaces inferieurs et I'Homme, et 

 meme entre eux et les Fourmis ; tandis que nous 

 voyoDS les differentes couleurs et leurs differentes 

 intensites, les Crustaces inferieurs ne voient qu'une 

 seule couleur dans ses differentes variations d'intensite. 

 Nous percevons des cou'eurs comme couleurs ; ils ne 

 les perpoient que comme lumiere." * 



It is by no meaus easy to decide such a question 

 absolutely ; but the subject is of much interest, and 

 accordingly I made some further experiments, as it did 

 not seem to me that those of 31. iMerejkowsky bore out 

 the conclusion he has deduced from them. 



Professor Dewar most kindly arrauged the apparatus 

 for me again. He prepared a normal diffraction-spec- 

 trum, produced by a Rutherfurd grating with 17,000 

 lines to the inch ; the spectrum of the first order was 

 thrown on the trough. In this case the distribution of 



* M. C. Merejkowsky, " Les Crustaces inferieurs distinguent-ils les 

 couleurs? " 



