APPARATUS TOR EXHIBITING COLOURS. 283 



lion, were rendered as fure and inviolable, as they are at 

 prefent the contrary, they would ftill have this inconvenience: 

 they are liable to be forced by introducing Tome tool or inftru- 

 ment ; and even if the violator fhould fail in his attempt, the 

 proprietor will never thelefs be reduced to the neceffity of 

 breaking open his own door if the intide of the lock fhould be 

 ever fo little deranged. The advantage then of a lock which 

 affords no entrance to a key is obvious, a problem which I 

 think cannot be well folved, except by a circular motion. 

 That which I have defcribed, not only anfwers all the condi- 

 tions of the theory, but has flood the teft of experience. The 

 greateft obftacle to the propagation of locks of combination, 

 proceeds from the prejudices of habit, ftill more than from 

 their want of fimplicity : and it is well known that this is the 

 fate of all truths *. 



x. 



Defcription of Dr. Young's Apparatus for exhibiting the Co* 

 lours of thin Plates, by means of the Solar Microfcope. From 

 the Journals of the Royal Inftitution, I. 241. 



JL HE colours of thin plates were obferved by Boyle and The colours of 

 Hooke, and more accurately analy fed by Newton : but little SjHfJj 

 or nothing was added to the account that Newton gave of mined, and ne- 

 them, until fome attempts were lately made to explain them, j*j. P" bllc, y e *- 

 and to build at the fame time on the explanation, the principal 

 arguments in favour of a new fyflem of light and colours. The 

 phenomena themfelves were very little known, except from 

 Newton's defcription ; it had happened but to few to obferve 

 them : and they had never been made confpicuous to a public 

 audience in a form equally beautiful and interefting. 



It appeared, however, that there w r cnild be little difficulty New attempt t» 

 in applying the apparatus for reprefenting opaque objects i n exhibit them by 

 the folar microfcppe, to the exhibition of thefe colours on a CO p e% 

 large fcale : but feveral precautions were neceflfary, in order 

 to obtain the moft advantageous reprefentation ; and, thefe 

 precautions having been completely fuccefsful, it may be of 

 fome utility to give a detached account of them. 



* In the former Paper for Requier read Reignier, and in the 

 note on p. 217, for Braviali read Bramah. 



The 



