174 P. C. on the Colours that enter into the [Sept. 



a sufficient separation of the two reflections, but it is open 

 to the objection, that the reflection from the second surface 

 of the coloured glass, is mixed with the reflection from the 

 first surface : in practice, however, I have never discovered 

 any inconvenience from the superposition of the two reflec- 

 tions; the accidental colour being, as far as I can judge, 

 precisely the same as that obtained by placing the coloured 

 glass close to the mirror. This, though at first view it 

 appears to be attended with some difficulty, is perfectly 

 consistent with the principles we have advanced ; the eye, 

 under the influence of a stronger impression of the same 

 kind, is insensible to the smaller quantity of coloured light 

 within the shadow.'^ 



I have since improved upon this method, by placing the 

 coloured glass upon a mirror, inclined towards the window, 

 as in the last experiment, and by then raising the edge of 

 the glass nearest the window, until the two images oiF the 

 finger, placed so as to form a shadow, are sufficiently sepa- 

 rated : the object being to reflect the light from the mirror 

 through that part of the coloured glass from which the 

 direct light is excluded, is thus readily accomplished. 



• The complementary colours sometimes exhibited by shadows, in coloured 

 rooms, may be explained upon the same principle. 



The room in which I am sitting is painted a reddish drab ; and there being two 

 windows in it, at some distance from each other, objects placed to intercept the 

 light, form two shadows upon the opposite wall ; both these shadows are, of course, 

 enlightened by one window only, and I have frequently been struck with the blue 

 or rather, violet appearance of shadows, thus partially illuminated, when my mind 

 has been occupied with subjects of a different character. The eye, in this case, 

 impressed with the light of both windows, reflected from the coloured walls, is 

 not only insensible to the feeble excess of colour within the shadow, but, also, to 

 the light of the same kind, which enters into tlie composition of the white light 

 that accompanies it. I have generally observed that, to a certain extent, the less 

 the shadow is enlightened, the more distinctly it appears of the complementary 

 colour of the room. 



To satisfy myself of the correctness of this explanation, I received the shadow 

 of an object partly upon a sheet of white paper, and partly upon the wall against 

 which the paper was placed ; upon examining the divided shadow I could discover 

 no difference in the accidental colour produced under these different circumstances, 

 except, perhaps, a little more distinctness of colour in that part of the shadow 

 which fell on the white paper. When the accidental colour is not seen upon first 

 viewing the shadow, it may generally be discovered by looking at the enlightened 

 wall and the shadow, in succession. 



Probably the vivid appearance of accidental colours, when the eyes are closed 

 after being impressed, may be attributed to the same cause : viz. the insensibility 

 of the eyes to colours so suddenly and greatly reduced in intensity. 



