117 



the intestine is neural, and they take a very natural position among 

 the neural mollusks between the Polyzoa on the one hand, and the 

 Lamellibranchs and Pteropoda on the other. 



The arms of the Brachiopoda may be compared with those of the 

 Lophophore Polyzoa, and if it turns out that the so-called hearts 

 are not such organs, one difference will be removed. 



In conclusion, I may repeat what I have elsewhere adverted to, 

 that though the difference between the cell of a Polyzoon and the 

 shell of a Terebratula appears wide enough, yet the resemblance be- 

 tween the latter with its muscles and the Avicularium of a Polyzoon, 

 is exceedingly close and striking. 



VIII. "An Inquiry into some of the circumstances and prin- 

 ciples which regulate the production of Pictures on the 

 Retina of the Human Eye, with their measure of endurance, 

 their Colours andChanges." Part II. By theRev. WILLIAM 

 SCORESBY, D.D., F.R.S., Corresp. Inst. of France, &c. 

 Received May 31, 1854. 



This second part of the author's inquiries concerning phenomena 

 in optical spectra, embraced the results in respect of colour in the 

 images impressed on the retina, as derived simply from the influence 

 of light. 



The optical spectra from white, grey, or black opake objects under 

 faint illumination, or of ordinary windows or apertures transmitting 

 low degrees of light, were usually found to be without colour. But 

 ordinary daylight, and, much more, the light from bright sunshine 

 (as is well known), yield chromatic spectra of vivid or brilliant hues. 

 By viewing with slightly closed eyes, the pictures impressed on the 

 retina by a few seconds' steady gazing at some fixed point of an 

 illuminated object, and noting the various effects, disappearances and 

 changes, a considerable number of characteristic phenomena were 

 elicited, and the effects of a variety of modifying circumstances satis- 

 factorily determined. The most prevailing influences in modifying 

 the phenomena whatever other causes might tend to the produc- 

 tion of variation in the colours were found to be referable to differ- 



