ON COLOURS or THIN PEtHCLES. ISQ 



tints, sometimes very -brilliiint. It is hot my intention here 

 to describe these, still less to dispute the particulars so ad- 

 mirably described by Newton. 1 shall only attempt to draw- 

 some conclusions respecting the origin of these colours, to 

 establish a comparison with those arising from absorption, 

 and to assign the trus cause of some phenomena hitherto 

 diil'ereritly explained. 



The principal effects, to which it is of importance for these 

 purposes to call the attention, are the following. 



When the light falls on very thin bodies, that exhibit the When light 



.• - 1 fallo oil theses 



prismatic colours : 



1. At the places where tliese colours arise on the thin sub- it is partly re- 

 stance, each pencil of rays, or if you please the white light, transmitted'^' ^ 

 is separated into two portions in a variable manner, and one 



of these portions is reflected, while the other can issue from 

 the substance only bj transmission. 



2. This division of the pencil varies according to a cer- This depends 

 tain iaw, which depends on the thickness of the body, its ^^^^l^y ^and^^* 

 density, and the inclination of the luminous rays, inclination to 



3. Each ray in particular comports itself, as if it possessed p , , 

 the singular property of having fits of easy reflection at pe- alternate fits of 

 riodical intervals, and fits of easv transmission at other in- easy reflection 



. , 1 » * n-«i • ^"" transmis- 



tervals alternating with the former. 1 hese various results sion. 



are equally indisputable. 



But whence can this disposition of the rays arise ? Newton Newton sup- 

 has considered it as inherent. in the rays themselves, not only of this inherent 

 in that part of their passage comprised between the two ex- in the ray itself, 

 treme surfaces of a body that they traverse, but throughout '^ ^"^ ^ *^^* 

 the whole course of these rays, from the moment they begin 

 to issue from a luminous body*. This is a kind of occult 

 cause, of which it is difficult to form a clear idea; and ac- 

 cordingly some distinguished philosophers have shown great 

 hesitation to admit it. 



But Newton himself, at. the end of his work, puts us into ^^ another re^ 

 the right road in a more bappy manner, when heasks, whe- fers it to the 

 ther it be not by virtue of the same principle, that the rays ifg^eflection, 

 are reflected and refracted by bodies, and inflected in their 

 ricinity f. 



• Opt. lib 2, part 3, prop. 13. -f lb. lib. 3, qujest, 4. 



Vol. XVIII—OcT. 1807. K It 



