ID* Sir D. Brewster on the Colours of Mixed Plates. 



lens placed at a greater distance from the diffracting edge than 

 its principal focus, the middle of the system of fi'inges corre- 

 sponding to the diffracted shadow of a fibre is occupied with 

 the direct tint, which we shall suppose to be green ; and on 

 each side of this green shadow, as we may call it, we observe 

 very faintly the complementarij red tinging what are called the 

 two first exterior fringes. This tinee of red is strongest in the 

 first fringe within the solid edge, or within the green shadow, 

 while it is yellowish in the first fringe without the green sha- 

 dow. These effects are inverted if we place the lens nearer 

 to the edge than its principal focus. 



The phaenomena now described appear more distinct if we 

 take an extremely narrow piece of nacrite, having its two edges 

 nearly in contact, and transmitting only a narrow line of light. 

 In this case the two red fringes within the solid edge unite 

 their tints, and become a bright red ; and in like manner if 

 •we place the lens nearer the solid edges than its principal fo- 

 cus, the two yellow fringes will nnite their tints, and become a 

 brighter yellow band. In this last case, when the two bound- 

 ing edges are still nearer each other, the united fringes, in 

 place of being yellow, will be green, or the same as the direct 

 colour. 



If we bring the edges of two pieces of nacrite of equal thick- 

 ness very near each other, having, as formerly, green for the 

 direct, and red iov \h& complementary colour, the space between 

 the edges, or between the green bands, will be faint red when 

 the lens is nearer the edges than its principal focus, andyel- 

 loxv when it is further from them ; but if the edges are brought 

 still nearer, the faint red will become brighter, and the united 

 green bands will take the place of the yellow one. 



Let us now return to our plate of nacrite with a single edge, 

 having gi-eeti and red for the two tints : and let us always sup- 

 pose that the lens is adjusted to observe the diffracted fringes, 

 that is, that the lens is placed at a greater distance from the 

 diffracting edge than its principal focus. We shall also sup- 

 pose that the light of the sun passing through a narrow aper- 

 ture parallel to the diffracting edge is substituted for the light 

 of a candle. Under these circumstances the central part of 

 the system of fringes seen by light incident perpendicularly, 

 consists of blue*, green, and yellow light, constituting, as it 

 were, the shadow of the edge, the blue light being on the 

 same side as the plate of nacrite, and the yellow rays encroach- 

 ing upon the exterior faint red band already described, the 

 other red band next the blue being more distinctly seen. If 



• Owing to the small quantity of blue rays in candle-light the blue almost 

 disappears in it. 



