471 Cemettt for wiJe-motrthei FeJJils.—Eyes of Birds. 



I avoid entering into any theoretical obfervations on the aforefaid phenomenon, until 

 more fully illuftratcd liy experiment, and content myftlf with merely announcing it to the 

 curious inveftigators in the paths of philofophical botnny. 



I take this opportunity of communicating another method, or at lead another compo- 

 (ition for cementing wide-moutlied vcflels, in addition to thofe mentioned in your Journal 

 for September. It is a mixture of fpermaceti and caoutchouc : the former to be n. cited 

 in a ladle, and the latter added in fmall bits, which will- be gradually but efftftually dif- 

 folved, and the compound forms a cement pcrfei£tly air-tight when poured on Huid and 

 fuffercd to cool. It is alfo, I believe, very little, if at all, attacked by acids, and on that 

 account might be of confiderable ufe in philofophical laboratories. The quantity of caout- 

 chouc may be varied according to the intended purpofe, or any hardening fubftance, at 

 maftic, added if required. If this latter communication (hould prove of any ufe to you 

 perfonally, or to your chemical friends, it will amply gratify 



Your obliged reader, 

 Die. ij}, 1797. AMICUS. 



IX. 



Experimental Rifearches to afceriaiit the Nature of the Procefs by which the Eye adapts itfelf 

 to produce dijiincl Vifion. 



[Continued from page ^13.] 



J[N the former part of this article, refpe^iing the adjuftment of the eye, a confiderable 

 number of original experiments and remarks were related from communications to the 

 Royal Society. Thefe have for their obje£ls the determination, whether the crytlalline hu- 

 mour by its fuppofed mufculaiity, the external mufcles by their greater or lefs prcflure up- 

 on the orbit, or the cornea by a variation in its curvature, be each the fole or chief agent i|i 

 caufing the pencils of light from vifible objeCls to arrive in all cafes at thofe foci which 

 diftinft vifion requires. I fliall now proceed to give the fubftance of the other Memoirs 

 publilhed by the Royal Society upon the fame fubjc£l 



In the volume for 1795, page 263, I find a paper of Obfervations on the Eyes of Birds, 

 by Mr. Pearce Smith. Thb gentleman relates, that, in the year 1792, he obfen-ed, while 

 «li{re£ling the eyes of birds, an irregular appearance of the fclerotica in that part which fur- 

 rounds the cornea, and which in them is general y flat. On a more minute examination, it 

 appeared to be fcales lying over each other, and which appeared capaWe of motion on 

 each other. On inveiligating this ftruclurc, the fcales were found to be of bony hardnef;, 

 at leaft much more fo than any other part of the fclerotica. On the inCde of the fclerotlc 

 coat there was no appearance of thefe fcales. Tendinous fibres were dete£\ed fprcading 

 over the fcales, terminating in the four re£li mufcles of the eye, fo that upon the contract 

 tion of thofe mufcles the fcales would be moved. 



By refle£ling on the probjble ufes of this conformation, Mr. Smith deduced, that the in- 

 ternal capacity of the eye will be grcated when the re£li mufcles are not in aiflion; and 

 the fcveral circles of fcales being fufTereil to repofe upon each other in fucceflion, are upon 

 5 the 



