in which Natural Bodies exhilit Colours. 203 



ed, Is it the product of a mixture of two heteroo-eneous 

 -substances ? and ia this case^ what are the conipounding 

 parts ? 



3d, Does light consist or not in a mixture of seven dif- 

 ferently coloured ravs ? and, if the latter be the case, how 

 and under what circumstances does light act as a medium 

 for the production of colours ? 



4th, What iufiucnce have the matters which in contact 

 with light produce colours on the change of its mixture? 



In regard to dnswering the first question, it will be ne- 

 cessary to detcruiine in general, if possible, whether any- 

 thing can exist actually perceptible to the senses in the 

 simple slate. 



As far as our experience in the knowledge of nature per- 

 mits us to judge, we must admit among material objects iu 

 general, and among the more subtle matters in particular, 

 an incessant- and mutual aciion, which is so powerful, that, 

 in the moment when we endeavour to disengage simple sub- 

 stances from their mixtures, they again enter uiio new com- 

 binations, and exercise on each other a productive power 

 which never ceases. 



It hence follows, that the impression by v/hich these 

 matters affect our senses must be considered either as the 

 rcsult of new mixtures ; or it must be admitted that these 

 jnatters, by their mixing and productive power, are capable 

 of excitine ideas of their specilic and individual existence. 



If this njode of considering the subject be applied to light, 

 it follows, that it must be considered as the result of the 

 mixture of two different component parts. But here a 

 question naturally arises, W hat are the principles which 

 produce lic'ht, considered as the product of their m.ixture ? 



As the answer to this question cannot be founded on any 

 question a priori, it must be explained by experiment 3 for 

 it is the result of a research respecting light that can pave 

 the wav to a solution of it. With this view the foUov.ing 

 experiments were made: 



Fj.rperimcnt I. 



On a bright sunnnerday, when the horizon was obscured 

 by no cloud, a bundle of rays was introduced into a darkened 

 room in such a manner as to cover the bulbs of two ther- 

 mometers suspended in a perpendicular direction: one of 

 tlicse thermomelerfi, which I shall call A, was at the di- 

 stance of 1L» inches, and the other, B, at the distance of -TA, 

 reckoning from the point where the light flowed in. 



Oiscrvution. — In the course of ten minutes the mercury 



