1882.] 



On the Refraction of Plane Polarised Light. 



393 



far tlie posterior part of the nervous system of the Bilateralia can be 

 regarded as derived from the primitive radiate ring. 



" A circumoral nerve-ring, if longitudinally extended, might give 

 rise to a pair of nerve-cords united in front and behind, — exactly such 

 a nervous system, in fact, as is present in many Nemertines (the 

 Enopla and Pelagonemertes) , in Peripatus and in primitive mol- 

 luscan types (Chiton, Fissurella, &c). From the lateral parts of 

 this ring it would be easy to derive the ventral cord of the Chastopoda 

 and Arthropoda. It is especially deserving of notice, in connexion 

 with the nervous system of the above-mentioned Nemertines and 

 Peripatus, that the commissure connecting the two nerve-cords 

 behind is placed on the dorsal side of the intestines. As is at once 

 obvious, by referring to the diagram (fig. 231 B), this is the position 

 this commissure ought, undoubtedly, to occupy if derived from part 

 of a nerve-ring which originally followed more or less closely the 

 ciliated edge of the body of the supposed radiate ancestor." " Com- 

 parative Embryology," vol. 2, pp. 311, 312. 



The facts of development here recorded give a strong additional 

 support to this latter view, and seem to render possible a considerable 

 extension of it along the same lines. 



IV. " On the Refraction of Plane Polarised Light at the Surface 

 of a Uniaxal Crystal. II." By R. T. Glazebrook, M.A., 

 F.R.S., f Fellow and Lecturer of Trinity College, Demon- 

 strator in the Cavendish Laboratory", Cambridge. Received 

 December 4, 1882. 



A paper of mine bearing the above title was read before the Royal 

 Society in November, 1881, and has since been printed in the " Phil. 

 Trans.," Part II, 1882. An abstract appeared in the " Proc. Roy. 

 Soc," No. 216, 1881. 



While continuing with the same apparatus a series of measurements 

 of similar nature, which have occupied me for the greater portion of 

 the present year, I have just discovered a source of error which had 

 hitherto escaped my notice, and which seriously affects all the results 

 I have arrived at. I have been using a spectrometer made many 

 years since by Grubb, of Dublin, for the late Dr. Robinson, of 

 Armagh, and kindly lent to me by Professor Stokes. 



A chance observation has showed me that the object-glasses of both 

 collimator and telescope in this instrument are strongly doubly 

 refracting. 



If plane polarised light fall on either, the emergent beam is ellip- 



2 d 2 



