6 An Account of the Repeating Ci/cle, 



not acquainted with the formation of two spectra when the 

 r?vys of light were made to fall perpendicularly on one of its 

 plane sides ; but I should suppose that he considered this as a 

 self-evident corollary of the general principle of refraction. 

 Indeed, if Newton had mentioned this as a particular disco- 

 very, he might as well have told us that when two opaque 

 bodies were intei-posed between the sun and a wall, there 

 were also formed two shadows. Dr. R. says that " mathema- 

 ticians are here obliged to relinquish one of their favourite laws, 

 that rays striking at right angles to plane surfaces suffer no 

 refraction ;" but he will here be pleased to recollect, that when 

 rays fall at right angles to one of the faces, they must strike 

 either of the other faces obliquely, and consequently be re- 

 fracted at their emergence. 



I should consider it an idle task to proceed any further in 

 the refutation of doctrmes which do not carry along with them 

 any thing like demonstrative evidence, but hinge entirely on 

 the author's own ij)sc dixit ; my principal object being only to 

 show the inconsistencies wliich result from the rejection of the 

 law of refraction. 



The Doctor has requested me to read his paper on Vision, 

 published in a former Number of your Magazme ; but I sup- 

 pose he must mean that which he published some tune ago in 

 " The Annals of Philosophy," which I have also read ; but 

 consider it quite foreign to the matter in question, whether the 

 ideas of visible objects be conveyed to the mind by retinal or 

 corneal images. I am, sir, 



Your most obedient servant, 

 H*K Majesty's Ship Queen Charlotte, CharlES StarK. 



Portsmouth Harbour, May 26, 1822. 



II. An Account of the Repeatiiig Ci7xle, and of the Altitude 

 find Azimuth Instrument; describing their different Construc- 

 tions, the Manner of performing their principal Adjustments, 

 and ho-iV to make Observations with them .• together with a 

 Comparison of their respective Advantages. By Edward 

 Troughton, Esq. F.R.S., and Member of the American 

 Philosophical Society*. 



KJv all astronomical instruments, those fixed in national obser- 

 vatories must be considered of the first importance to science; 

 and in a commercial country, like our own, perhaps those sub- 

 servient to nautical astronomy ought to be regarded as the next 

 point in of utility. Those which I would call the third class are 



* From the Memoirs of the Astronomical Society of London. 



jiumcrous ; 



