TOL. LXXXVI.3 PHILOSOPHICAL TRANSACTIONS. 731 



Stead of the pin ; also, they were much brighter in direct than in reflected light, 

 and in the light of the sun at the focus of a lens, than in his direct unrefracted 

 light. Placing a piece of paper round the hole in the window-shut, I observed 

 the colours continued there; and inclining the chart to the point where they left 

 off, I saw them continued on it, and then proceed as before to the shadow. If 

 the pin was held horizontally, or nearly so, they were seen of a great size on the 

 floor, the walls and roof of the room forming a large circle ; and if the chart was 

 laid horizontally, and the pin held between the hole and it, in a vertical position, 

 the circle was seen on the chart, and became an oval, by inclining the pin a little 

 to the horizon. 



Obs. 1. Having produced a clear set of colours, as in the last observation, I 

 viewed them as attentively as possible, and found that they were divided into sets, 

 sometimes separated by a gleam of white light, sometimes by a line of shadow, 

 and sometimes contiguous, or even running a little into each other. They were 

 spectra, or images of the sun, for they varied with the luminous body by whose 

 rays they were formed, and with the size of the beam in which the pin was held ; 

 and when, by placing it between the eye and the candle, a little to one side, I let 

 the colours fall on my retina, I plainly saw that they resembled the candle, in 

 shape and size, though a little distended, and also in motion, since if the flame 

 was blown on, they had the like agitation. The colours therefore which fell on 

 the chart were images of the sun ; they had parallel sides pretty distinctly defined, 

 but the ends were confused and semi-circular, like those of the prismatic 

 spectrum. Like it too, they were oblong, and in some the length exceeded the 

 breadth 6, or even 8 times ; the breadth was, as I found by measurement, exactly 

 equal to that of the sun's image received on a chart, as far from the pin as the 

 image was, and the length was always to the breadth at all distances, in the same 

 ratio, but not in all positions of the pin ; for if it was moved on its axis, the 

 images moved towards the shadow on one side, and from it on the other, be- 

 coming longer and longer (the breadth remaining the same) the nearer they came 

 to the shadow on the one side, and shorter in the same proportion, the farther 

 they went from it on the other. 



Obs. 3. Having picked out an image that appeared very bright and well de- 

 fined, I let it through a hole with moveable sides, in the upper part of a sort of 

 desk, which moved to any opening by hinges, and had a chart for its under side, 

 on which the image fell, and I shut the hole so close as to prevent any of the 

 others from coming through ; I then had a full opportunity of examining it, in 

 all respects, and I counted in it distinctly the 7 prismatic colours ; the red was 

 farthest from the shadow of the pin, and from the pin itself; then the orange; 

 then the yellow, green, blue, and indigo, and the violet nearest of all ; in short, 

 it was exactly similar to a prismatic spectrum, much diminished in length and 



5 A 2 



