This is definitely an instrument for the research laboratory , and demands considerable manipulative skill if reliable results are to be obtained . It may be considered as two spectroscopes, the other into the bottom, and the two spectra can there fore be compared with each other.
The pattern under examination is illuminated by a standard white light, which is obtained by passing the light from a 1,000 -watt globe through colour filters wich correct its colour to that of natural day light.
The light reflected from the pattern passes through one of the spectroscopes, where it is splot up into a spectrum , and this thrown into the top half of the eyepoece.
The second spectroscope is illuminated by light from a Pointolite lamp, and provides, in the lower half of the eyepiece, a standard spectrum against which the spectrum provided by the light from the pattern is compared.

            There are thus two spectra arranged one above the other like two ribbons.  By means of shutters in the eyeiecee, a narrow region of both spectrea is isolated. In the case of a green pattern under examination , if the red ends of the spectra are isolated, then the re din the spectrum from the green pattern will be very faint, whilst the red in the spectrum from the pointolite lamp will be intense.
By suitable means the brightness of the red in the standard spectrum is reduced until it is an exact match with the red in the spectrum from the pattern. The intensity of the nest portion of the spectrum, which is orange red  .
This procedure is repeated with perhaps forty narrow wave length bands and is obviously a very laborious matter.
Eventually a set of readings is obtained which can be plotted to give a curve showing the reflection of light of various solours from the pattern.
Such a curve is of considerable importance in scientific investigations of such matters as the fading of dyestuffs on exposure to light , because the in the depth of shade as well as the change in shade are both clearly indicated .The colour is expressed as X% white +colour for each wave length measured.