28 
ture, decorated with a gold or other frame that does not overshadow 
it, perhaps rather in the shady corner of a room to preserve it, and 
when used, so placed that it may be nearly perpendicular so that the 
beams of light in the room, may illuminate the face of it; when a 
prism may be adjusted at a proper distance opposite to it. The 
prism may also have been selected, and perhaps measured as to its 
power of refraction, from which there may be a means of de- 
termining the distance or angle to be subtended, &c., in which case 
it will be most comjietent for use. 
One angle of the prism should be directed towards it, and the 
spectator should look in at the opposite face of the prism towards 
the lower face, where the spectrum will appear in great beauty and 
order. 
The upper separated wedge should be looked at by itself, and the 
lower one also. By use we may afterwards readily compare them, by 
moving the prism. 
The wedge or open chromatometer. Tab. "J. upper Jig. The 
square at the beginning will indicate the proper distance, by giving 
the spectator eight divisions or tints of regular and equal propor- 
tions, viz. 
1 Light Blue apparently above the black space. 
2 Darker Blue 
3 Violet 
4 Blue Black \ upon the black space. 
5 Black 
6 Scarlet 
\ below the black space. 
8 Lighter YellowJ ^ 
A broader square wiU not give more divisions of colours either 
regular or otherwise, if observed either at the same or at a less 
distance. 
The commencement of the wedge should give two parts of blue, 
one of indigo, one of violet, one of red, and two of yellow, nearly as 
Tab. 3. i. or as Tab. A. d. The dark blue, indigo, and violet will 
