73'2 PHILOSOPHICAL TRANSACTIONS. [aNNO IJQi). 



breadtli, and turned horizontally on the wall opposite to the prism, with the red 

 farthest away. In fig. 5, se is the pin, reflecting the rays cp and co, vvliich pass 

 through po, the hole in the desk, ed, to the chart or bottom of the desk, ktsd, 

 and form there the spectrum ik divided into its colours, i being violet, and k red« 

 On moving the hole in the desk, and letting through other images, the colours 

 were not in all arranged the same way, but I moved the pin on its axis, and ob- 

 served those where the order was inverted to move, not only with respect to the 

 pin, but also with respect to the contiguous images ; and I was surprized to see 

 them assume the order of colours first mentioned, namely, the red outermost, 

 and the violet innermost. In like manner the images, which before the motion 

 were regular, on moving into the places left by the others had always the order 

 of their colours inverted, so that the thing must be owing to some irregularities 

 in the pin's surface ; for those which were made by a small glass tube filled with 

 quicksilver, and freed from scratches by a blow-pipe, preserved during the motion 

 the proper order of colours. Another irregularity in the arrangement was also 

 observable even in the glass tube ; for 2 contiguous images, by mixing one with 

 another for 2 or 3 successions, appeared each to have outermost a dull colour, 

 between red and violet, and innermost a green ; but here, unless the succession 

 continued through all the images, the outermost of all was red, and the inner- 

 most image had universally violet in the inside. 



Oki. 4. I placed at a hole in the window-shut a prism, to refract the rays, and 

 received the spectrum at the distance of 6 feet from the window, on a chart ; 

 then, at the distance of 2 feet, I placed a screen with a hole in the middle of it, 

 through which I let pass successively the ditferent rays. At the distance of 1 

 inch from the hole, between it and the chart, I placed the reflecting cylindrical 

 body ; the images were found on the chart and walls of the room round to the 

 sides of the hole on the screen, and were always wholly of the colour in which 

 they were formed, except in the confines of the green, where a small quantity of 

 white light fell, and made them of all the 7 colours ; but this was almost wholly 

 prevented by using a prism with a greater refracting angle, and holding the pin 

 and screen farther from it. I then removed the screen, and left the reflector in 

 its place, so as it might reach through the rays ; and thus there were formed 

 images, having in them, from top to bottom, the 7 colours, one after another, 

 the lowest division being red, the highest violet. They were inclined considera- 

 bly towards their tops, and were much broader at the bottom or red parts than at 

 the tops or violet parts. And lastly, the reflector being moved so that the images 

 might be disturbed, as in the former experiment made in the white light, the 

 red was most, the violet least dilated. In case these eflects might be owing to 

 any peculiarities in the shape or position of the reflector, I placed at 3 feet from 

 the prism a lens of 4 inches breadth, to collect the rays to a focus, 6 feet beyond 



