TRANSIT OF MERCURY. gj 



pretty well afTured, that the alterations of the focal length muft Experiments on 

 be owing to a dilatation of the glafs or metal of which mirrors ^ i )e^"\h i 

 are made, and muft be greateft where moft heat is applied, mirrors by heat. 

 Our experiments therefore cannot agree perfe611y with folar 

 obfervations ; for, in the glafs mirror, the application of partial 

 heat in front, muft undoubtedly have been much ftronger about 

 the middle of the mirror (though the centre of it was fome- 

 tim-'^s guarded by a brafs plate equal to the fize of the fmall 

 fpeculum) than at the circumference. But w hen, on the con- 

 trary, a mirror is expofed to the fun, every part of the furface 

 will receive an equal portion of heat. 



It may alfo be laid, that I have pointed out a defed in tele- 

 fcopes ufed for folar obfervations, without affigning a cure for 

 it. It will however be allowed, that tracing an evil to its caufe 

 muft be the firft ftep towards a remedy. Had the imperfec- 

 tion of the figure brought on by the heat of the lolar rays been 

 of a regular nature, an elliptical fpeculum might have been 

 ufed to counteract the alTumed hyperbolical form ; or vice 

 terfd. 



And now, as, properly fpeaking, the derangement of the 

 figure of a mirror ufed in obferving the fun, is not fo much 

 caufed by the heat of its rays as by their partial application to 

 the reflecting furface only, which produces a greater dilatation 

 in front than at the back, there may be a potTibility of counter- 

 acting this etfe6t, by a contrary application of heat agau)ft the 

 back, or by an interception of it on the front. But this we 

 leave to future experiments. 



III. 



Obfervations on the Chemical Nature of the Humours of the Eye, 

 jBy Richard Chenevix, Efq, F.R. S.and M. R. LA* 



J. HE functions of the eye, fo far as they are phyfical, have 

 been found fubjedl to the common laws of optics. It cannot be 

 expected that chemiftry fliould clear up fuch obfcure points of 

 phyfiology, as all the operations of vition appear to be ; but, 

 fome acquaintance with the intimate nature of the fubftances 



* From the Philofophical Tranfaaions, 1803. 



which 



