*Rtr. 



.] DISSERTATION SECOKD. 73 



colours. 1 There is not, perhaps, in science any happier ap- 

 plication of theory, or any in which the mind rests with ful- 

 ler confidence. 



Other meteoric appearances seemed to be capable of simi- 

 lar explanations, but the phenomena being no where so 

 regular or so readily subjected to measurement as those of 

 the rainbow, the theory cannot be brought to so severe a 

 test, nor the evidence rendered so satisfactory. 



But a more difficult task remained, — to explain the perma- 

 nent colour of natural bodies. Here, however, as it cannot 

 be doubted that all colour comes from the rays of light, so 

 we must conclude that one body is red and another violet, 

 because the one is disposed to reflect the red or least refran- 

 gible rays, and the other to reflect the violet or the most re- 

 frangible. Every body manifests its disposition to reflect the 

 light of its own peculiar colour, by this, that if you cast on it 

 pure light, first of its own colour, and then of any other, it 

 will reflect the first much more copiously than the second. 

 If cinnabar, for example, and ultra-marine blue, be both ex- 

 posed to the same red homogeneous light, they will both ap- 

 pear red ; but the cinnabar strongly luminous and resplen- 

 dent, and the ultra-marine of a faint obscure red. If the 

 homogeneal light thrown on them be blue, the converse of 

 the above will take place. 



Transparent bodies, particularly fluids, often transmit light 

 of one colour and reflect light of another. Hallev told New- 

 ton, that, being deep under the surface of the sea in a div- 

 ing-bell, in a clear sunshine day, the upper side of his hand, 

 on which the sun shone darkly through the water, and 

 through a small glass window in the diving-bell, appeared of 

 a red colour, like a damask rose, while the water below, and 

 the under part of his hand, looked green. 3 



1 Optics, Book I. prop. 9. 2 Optics, p. 115. Horseley'sedit. 



10 



