Exptrimenis on the focal AJjujtw.cnt of the E)t. 30 - 



more fimpls, to confider light not as a diftintl fpccies of matter, but as a il.ite of caloric, which 

 is manifefted by its producing the fenfation termed vifion. It is demonllrable alfo, that the 

 ponderable parts of oxygen and hydrogen gaz conditute water. There is ftrong evidence 

 that thefe gazes confift of a peculiar fpscies of matter which is ponderable; and of im- 

 ponderable matter, which is that which is feparable from them in the ftate of fire, or flame. 

 If fire could be applied in a fufficiently denfe flate and quantity, it is warrantable, from a 

 full induaion of fads, to conclude, that it is able to difunite the conftituent fubftances of 

 all the compound fubftances in nature. 



[To be concluded in the next Niitiibcr.'} 



IV. 



Experimental Refearches to a/certain the Nature of the Procefs by which the Eye adepu it/elf t, 

 ^^ produce diflinSl Vifoii. 



-I HE ftrudureof the eye, and particularly its provifions for adjuftment to produce 

 diftina vifion, have engaged the attention of feveral philofophers during the laft five years, 

 who have dehvered papers on thefe fubjeds to the Royal Society. I purpofe to give the' 

 fubftance of their difcoveries in the prefent communication. 



In the year 1793, Mr. Thomas Young's Obfervations on Vifion appeared in the Tranf- 

 adions. He gives a fhort fummary of the theories of adjuftment propofed by various earlier 

 authors. Kepler fuppofed the ciliary procefies to contraft the diameter of the eye, and 

 lengthen its axis by a mufcular power. Defcartes imagined the fame effea to be produced 

 by a mufcularity of the cryftalline humour, but did not attribute much to the change of 

 figure which he fuppofed to take place in that lens. De la Hire, and alfo Haller, adopted 

 the opinion that the eye undergoes no change to produce diftinft vifion, but the contradion 

 and dilatation of the pupil. Pemberton fuppofed the exiftence of mufcular fibres in the 

 cryftallme humour, by which the curvatures of its furfaces are changeable. Dr. Porterfield 

 conceived that the ciliary proceJes draw forward the cryftalline, and render the cornea 

 more convex. Dr. Jurin maintained the hypothefis, that the uvea, at its attachment to the 

 cornea, is mufcular, and capable of increafing the convexity of that humour, by its con- 

 traftion towards the axis of the eye. Mufthcnbroek conjedures that the relaxation of his 

 ciliary zone, which appears to be nothing but the capfule of the vitreous humour, permits 

 the coats of the eye to pufh forward the cryftalline and cornea. And, laftly, an elongation 

 of the axis of the eye has been fuppofed to be produced by the prefibrc of the external 

 mufcles, efpecially the two oblique mufcles; and, on the other hand, the mufcular adion 

 has been fuppofed to produce a contrary elled, namely, a contradion of the axis. ' 



In the enumeration of thefe refpedive hypothefcs, Mr. Young makes remarks tending to 

 their feveral rcf itation. I have not tranfcribcd them, principally hecaufc they chiefly 

 tend to (hew that cfleds of this nature dcfcrve, if pofliblc, to be fubmitted to adual exi 

 pcrimcnt. •> 



From the confideration of tlic wliole fubjcd, our author concluded that the rays of irglit 

 emitted by olijeds at a fmall diftance could only be brought to foci on the retina by 'a 

 Vol. I.-OCTOBUR 1797. Rr nearer 



