Our eyes are capable of seeing only a narrow spectrum of light.
The dark lines of the spectrum of sunlight, earliest noted by Dr W.
The spark spectrum of gold has been mapped by A.
But, to obtain an equally good result in the m th spectrum, the error must be less than I/m of the above amount.'
Let us suppose that the light is incident perpendicularly, and that the grating interval increases from the centre towards that edge which lies nearest to the spectrum under observation, and decreases towards the hinder edge.