Focal .1iijiijlms))t of the E^e.—Sulphuresiis Acid, 3 1 3 



of being elongated i-iith part of its diameter, contracting to its former lengdi immediately 

 upon being left to itfelf. 



3. That the tendons of the four ftraight mufcles of the eye are Continued on to the edge 

 of the cornea, and terminate, or are inferted, in its external lamins : their a£lion will there- 

 fore extend to the edge of the cornea. 



4. That in changing the focus of the eye from fesrlng with parallel rays to a near dif- 

 tance, there is a vifible alteration produced in the ilgure. of the cornea, rendering it mors 

 convex ; and when the eye is again adapted to parallel rays, the alteration by which the 

 cornea is brought back to its former llatc is equally vifible. 



The remahiing part of Mr. Home'slefture contains fome obfervations upon the mufcularand 

 elaftic powers, which by their oppofition produce fo curious an efFeiS in the adjuftmentof the 

 eye. The feparate aflion of the mufcles produces a change in the direftion of the axis of the 

 eye. A fmall force of contraftion in the whole fyftem will (teady the eye, and a greater fores 

 will comprefs the lateral r.nd pofteridr parts of the eye, and render the cornea more convex. 

 The experiments prove that the eye-ball cannot recede in its orbit by thefe adlions. Other 

 in fiances in the animal oeconomy, by means of which the expenditure of mufcular aftion is 

 faved by the operation of elaftic antagonifts, are alfo ilated by this author. The exertion 

 required to adjufl: the eye to near diftances, and the eafe with which it was adapted to re- 

 mote objefts, prove that the firft was a pofitive a£lion, and the fecond a relief. The de- 

 feel of elafticity inferred to arife from age is happily applied to explain the changes of 

 ■vifion which take place in advanced life. 



Two other communications to the Royal Society by this philofopher, and one by Mr. 

 Smith on the eyes of birds, remain to be confidered ; but, on account of the length of this 

 communication, I Ihall for the prefent defer them. 



Concerning the Properties of the Sulphureous Acid, and its ComUnations with earthy and alkaline 

 Safes. By Citizens FOURCROY and VaU^'ELIN *. 



. OEVER AL philofophers have paid attention to the properties of the fulphureous acid, 

 and fome of its combinations ; but no one has given a complete account of this acid. 



BerthoUet is almoll the only chemift who has opened this inveftigation. He is the firfl: 

 who publilhcd any accurate account of the fubjcft f. 



We fliall not in this place dcfcribe either the apparatus or method of preparing this acid, 

 bccaufc both arc well known to fuch as are moderately acquainted with chemiftry. 



/. Ph-,f:cal PropcrtiiS of the Sulphureous Acid. 



THIS acid conflitutes a permanent clnftic fluid at the ordinary preffure and temperature 

 of our atmofphcre. Its odour is Itrong and fuffbcating. - It cannot maintain combullion, 

 tior tlic rcfpiration of animals. 



r,cr/man, in his Trcatife on Elc£live Attraction, aflirms tliat tlic fulphureous acid, pre- 



■ Journal dc I'EcoIe Polyi«lini<ioe, c;.hii.t 1\ . p. 445. I Annalc.dc Cliimie, II. 54. 



A' I. 1. — OcTonr.R 1797. Sf pared 



