CONSIDERATIONS ON COLOURS. 121 



between the actual colour of the part enlightened, and that 

 of the part in (hade. 



Contrafts may Jikewife render the colour of the clouds com- Contrafts affcQi 

 plicated, as for inftance, when a great portion of the fky dif- clouds# 

 plays its blue tint. There are tome clouds, the colour of 

 which arifes folely from this caufe ; and fuch may be feen at 

 times in the middle of the day, when we have a lofty moun- 

 tain at our back, or are in any other iituation where the eye 

 is defended from the too powerful action of the folar light, 

 either direct or reflected ; but in this cafe the clouds have 

 only a yellowifh tinge, precifely the complementary colour 

 of Iky blue. 



Sometimes we fee the moon of a fimilar colour, when The Mooa. 

 it is very high, a little before or after the fun patfes the 

 the horizon : farther it appears thus, or even completely 

 white, when clouds varioufly coloured by the vapors of fun- 

 fet or fun-rife exift in the air at the fame lime. From this 

 concurrence of circumitances we have a new proof of the 

 difference ofcaufesto which thefe colours are owing. 



Laftiy let us remark, that from the irregularity of the earth's 

 furface, and of the ltate of the atmofphere, the phenomena 

 are liable to be concealed or fubjecled to various interruptions. 

 In our climate the colouring of the clouds feldom reaches its 

 lait llage. On fome evenings however, when the (ky is very r^ clouda 

 clear toward the part where the Sun fets, while light clouds ° ver head aC 

 float very high over our heads, we (hall fee thefe at a fub- 

 fequent period appearing of a very light red, heightened by 

 the diminution of light on the earth, foon after obfcured, and 

 at length becoming extinct in fhade. 



Conclufion. m 



Notwithfianding the many beautiful difcoveries already The theory of 

 made reflecting light, the theory of the production of colours ^ urs imper " 

 has not yet attained a degree of generalization that renders 

 it applicable to all cafes, or that fimplicity of principles to 

 which we are almoft always Jed when we have difcovered the 

 real laws of nature. Many phenomena have eluded ex- 

 planation, and that given of ieveral requires correction. Our 

 author has propofed to eftablifh alterations in the theory, the 

 lieceffity of which he points out. He fupports his principles 

 partly by the doctrine and facts generally admitted; partly 



