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refractive and dispersive Powers . 
The position of the prism in which the colours are most 
clearly divided, is when the incident light makes about equal 
angles with two of its sides. I then found that the spaces AB, 
BC, CD, DE, occupied by them, were nearly as the numbers 
16, 23, 3 6, 25. 
Since the proportions of these colours to each other have been 
supposed by Dr. Blair to vary according to the medium by 
which they are produced, I have compared with this appearance, 
the coloured images caused by prismatic vessels containing sub- 
stances supposed by him to differ most in this respect, such as 
strong but colourless nitric acid, rectified oil of turpentine, very 
pale oil of sassafras, and Canada balsam, also nearly colour- 
less. With each of these, I have found the same arrangement of 
these 4 colours, and, in similar positions of the prisms, as nearly 
as I could judge, the same proportions of them. 
But, when the inclination of any prism is altered so as to 
increase the dispersion of the colours, the proportions of them 
to each other are then also changed, so that the spaces AC and 
CE, instead of being as before 39 and 61, may be found altered 
as far as 42 and 58.* 
* Although what I have above described comprises the whole of the prismatic 
spectrum that can be rendered visible, there also pass on each side of it other rays, 
whereof the eye is not sensible. From Dr. Herschel’s experiments (Phil. Trans, for 
1800) we learn, that on one side there are invisible rays occasioning heat, that are 
less refrangible than red light; and on the other I have myself obseived, (and the 
same remark has been made by Mr. Ritter,) that there are dkewise invisible rays of 
another kind, that are more refracted than the violet. It is by their chemical effects 
alone that the existence of these can be discovered ; and, by far the most delicate test 
of their presence is the white muriate of silver. 
To Scheele, among many valuable discoveries, we are indebted for having first 
duly distinguished between radiant heat and light; ( Traite de V Air et du Feu, § 56, 
57 ;) and to him also we owe the observation, that when muriate of silver is exposed 
