250 Mr. Delaval on the Caiije af the 



in proportion as the paffage through their 

 interftices is obftruded; fome of thofe media, 

 during the t^ecreafe of their tranfmitted light, 

 do not vary the fpecies of their colour : but, 

 in other Tranfparent Coloured Media, the tranf- 

 mitted colours are changed, by the refiftance 

 to v/hich the rays are oppofed, during their 

 paflage through the Interftices. 



The changes, which are thus effected in Tranf- 

 parent Coloured Media, generally proceed in 

 one and the fame order. So that, in proportion 

 as the paflage of the more refrangible rays, 

 through fuch fubftances, is obftrufted, the lefs 

 refrangible rays, by their greater momentum, 

 penetrate the medium and are tranfmitted 

 through it. 



From the mixture of the lefs refrangible rays, 

 which are thus tranfmitted, the mafs exhibits 

 either a red, or fome colour inclining to red, 

 in proportion as a greater or lefs quantity of the 

 more refrangible rays are intercepted. 



It is probable, however, that fome inftances 

 may be found, of media whofe changes of colour 

 do not coincide with this general rule. Efpeci- 

 ally, if fuch media owe their colours to phlo- 

 giftic matter, or to particles which differ greatly 

 in denfity, from the pellucid part of the fubftances 

 through which they are difFufed. 



I have not met with any obfervations, rela- 

 tive to the changes of colour, arifing from the 

 \ faufe 



