the basic structure and operation of a spectrometer comprising a michelson type ft interferometer is discussed with reference to fig1 . the spectrometer 2 , by way of example only , comprises a michelson type ft interferometer 4 and an associated arithmetic unit 6 , which here is formed integral with the spectrometer 2 , operably connected to receive output from the interferometer 4 . it will be appreciated that the arithmetic unit 6 may be alternatively be provided as a separate but connected unit sited either locally or connected to receive output from the interferometer 4 via a network ( internet , lan , wan etc .) or indeed part of the functionality of the arithmetic unit 6 may be provided integrally , locally or remotely without departing from the invention as claimed the michelson type ft interferometer 4 typically comprises a stationary mirror 8 , a movable mirror 10 and a beamsplitter 12 ( typically also together with an associated compensator plate which for clarity is not illustrated ). a radiation source 14 , selected to emit radiation from within a wavelength region between ultraviolet and the infrared region of the electromagnetic spectrum , is arranged to launch radiation into the interferometer 4 via a collimator 16 and interfering radiation is directed through a sample cuvette 18 and towards an associated detector 20 . in other embodiments the sample cuvette 18 may , in use , be located in the path of the collimated radiation from source 14 at a position before the beamsplitter 12 but preferably at a position such that radiation from a reference energy source , here a solid state reference laser 22 , will also traverse it . as the movable mirror 10 is reciprocally translated radiation recombining at the beamsplitter 12 after reflection from the mirrors 8 and 10 will generate an interference pattern which is detected at the detector 20 to form an interferogram recorded in the arithmetic unit 6 . for each wavelength element of the radiation entering the interferometer 4 the beam components generated by the beamsplitter 12 will be completely in phase ( a maximum in the interference pattern ) at the beamsplitter 12 when the path length difference ( retardation ) between each mirror 8 , 10 is equal to a whole number , n , of wavelengths and will be completely out of phase ( a minimum in the interference pattern ) when the path length difference between each mirror 8 , 10 is equal to n + ½ wavelengths . between these two extremes the signal is dependent on the cosine of the retardation . the interferogram stored on the computer represents the intensity of the interference pattern at discrete points or positions of the movable mirror 10 . a typical interferogram 32 as may be recorded for milk is represented , for illustration only , in fig2 and , as can be seen , comprises a maximum amplitude at center - burst 34 and an oscillatory amplitude portion 36 having an amplitude envelope that decreases as retardation increases . the actual frequency composition of the interferogram is dependent on the wavelengths of radiation from the source 14 that reaches the detector 20 and so is dependent on the transmission properties of any sample material that is interposed in the beam path from source 14 to detector 20 . preferably when implementing the so called fast fourier transform ( fft ) analysis of the recorded interferogram the data is collected at equidistant points along the positional axis of the moving mirror 10 so that when the fft algorithm is applied in the arithmetic unit 6 the resulting spectral data will be represented by points equidistant in frequency . in order to obtain equidistant measurements of the interference pattern it is usual to employ an interference pattern generated using the reference source , here a solid state laser 22 . typically radiation from the laser 22 is directed into the interferometer 4 , such as via a dichroic mirror 26 , to follow substantially the same path through the interferometer 4 and sample cuvette 18 as the radiation from the source 14 and to be detected by an associated detector 28 . in the present embodiment the output from the detector 28 is passed to a zero - crossing detector 30 which is configured to provide trigger signals as , as discussed above , the cosine variations in detected intensity crosses a zero or other threshold level . this trigger signal is then employed to trigger a recordation of a data point in the arithmetic unit 6 or it may be recorded as an index of retardation at a location in the interferogram . in use the spectrometer 2 according to the present invention operates , in one embodiment , as follows and with reference also to fig3 . data representing a reference interferogram 40 is obtained into the spectrometer 2 and made accessible to the arithmetic unit 6 . this may be a ‘ master ’ interferogram that has been collected on a same type of instrument as the actual spectrometer 2 or may have been recorded by the spectrometer 2 . data representing a target interferogram 42 is then subsequently recorded by the interferometer 4 and is made accessible to the arithmetic unit 6 . the sample in the sample cuvette 18 that is used to generate the target spectrum should be the same or sufficiently similar to that used to generate the reference interferogram to , by itself , cause insignificant phase changes to the recorded interferogram . in one embodiment the sample material used to generate the reference and the target data may be a so called zero - liquid , typically water when making measurements on milk . the use of a zero - liquid is well known in the art of ft spectrometry in order to establish a background or ‘ zero ’ level relative to which transmission intensities for spectra of unknown samples are measured . interferograms of zero - liquid are recorded frequently , often between the recordal of successive unknown sample interferograms . zero - liquid interferograms may therefore be advantageously employed in the method and spectrometer according to the present invention since the normal operation of the spectrometer need not be interrupted to introduce samples specifically for use in the method according to the present invention . in other embodiments unknown samples of the same basic type , for example milk samples , may be employed in the generation of the reference and the target interferogram data . this is based on the realisation that the small compositional changes in the sample material for which ft interferometer based spectrometers are typically employed to monitor have only subtle effects on recorded interferograms which normally would not introduce significant phase changes . indeed since these compositional changes produce only subtle effects often the zero liquid and the unknown sample interferograms may be employed as the reference 40 and the target 42 interferograms . once data representing the reference 40 and the target 42 interferograms , in the present embodiment and by way of example only being interferograms obtained at different instances of the recordal of zero - liquid interferograms , is made accessible to the arithmetic unit 6 the arithmetic unit is configured to apply a phase correction to each of the reference 40 and the target 42 interferograms to ensure that for each of the interferograms 40 , 42 all contributing frequencies will be in phase at center - burst . in one embodiment the arithmetic unit 6 is adapted to determine the power spectrum ( the length of the complex spectrum after fft ). this will remove any phase difference between the frequency components that contributed to each of the recorded interferograms 40 , 42 . the unit 6 then applies an inverse fft to the so obtained power spectra to obtain symmetrical ‘ phase - corrected ’ reference 40 and target 42 interferograms in which each interferogram has zero phase shift of each contributing frequency at center - burst . by way of example , the arithmetic unit 6 may be configured to store the data element representing the intensity of center - burst for each interferogram 40 , 42 in an associated memory location indexed n = 0 and to store each consecutive data element at consecutively indexed associated memory locations . thus the index , n , of the associated memory location will have a correspondence with the position of the moving mirror ( or retardation ). the unit 6 is then operated to detect a phase shift between the phase - corrected interferograms 40 , 42 in a window of width w spatially ( or temporally ) disposed distal of the position of the center burst 44 , which center - burst is the same for each interferogram 40 , 42 . it will of course be recognised by the skilled person that the shift is actually a stretching of the interferogram but if determined in a relatively small window it can be described as a linear shift . it is useful but not essential that the window is located so as to measure a maximum shift with sufficient signal to noise . this can be achieved by operating the unit 6 to mathematically slide the window w along the time / distance ( here x axis ) of the interferogram , here by cycling through the indexes , n , of the associated memory locations in which the interferograms are stored , and measure positions ( index values ) along that axis of corresponding features , such as peaks , troughs or here zero - crossing positions , of the reference 40 and the target 42 interferograms and the related signal to noise ratios . alternatively , the position of the window w may be preset . once the location of the window w is decided , illustrated in fig3 as extending from index position n = a to n = b then a shift d between the determined positions ( for example index values , n ) of corresponding features , here illustrated as zero crossings , can be determined mathematically , for example by simple subtraction of associated index values , and may be applied directly to control the operation of the spectrometer 2 in order to generate data representing a frequency stabilized interferogram of an unknown sample recorded by the ft interferometer 4 . in a preferred embodiment however the arithmetic unit 6 is configured to employ the so determined shift , d , in an initial transform which is then applied to the target interferogram 42 . a phase correlation between the transformed target interferogram 42 and the reference interferogram 40 is then determined and the shift varied . a new transform is generated and the correlation is again measured . this may be repeated until a maximum correlation is determined . the shift at this maximum correlation , d max , is then employed in the spectrometer 2 to control its operation in order to generate data representing a frequency stabilized interferogram of an unknown sample obtained by the ft interferometer 4 . in one embodiment the so determined transform may be employed in the arithmetic unit 6 to mathematically phase compensate the interferogram data for unknown samples that is obtained from the interferometer 4 . in this embodiment the transform is applied to the so obtained data to move the entire interferogram an amount along the distance / time axis concomitant with the calculated shift d max . in a preferred embodiment this movement is determined as a fixed percentage ( or fraction ), the value of which is based on the shift d max , of the position on the interferogram of the point to be moved from center - burst 44 with index , n = 0 . zero crossings on a laser interferogram are essentially employed as a ruler to sample the interferogram at equidistant positions . drift in the operating frequency of the reference source 22 causes this ruler is to become stretched so that the interferogram at that time will actually be sampled at different positions with respect to an earlier recorded interferogram . preferably then , this stretching is measured as a percentage change of the distance between corresponding features of the two interferograms 40 , 42 . as the shift is very small it may conveniently be estimated as a fixed shift ‘ far from the centerburst ’— because here the small shifts has accumulated to a detectable shift . the detected shift , d , in the window , w , is then divided by the mean distance , using the nomenclature above may be given as ( b − a )/ 2 of the window , w , from the centerburst ( here x = 0 ) to calculate a percentage shift d % as : where z is the memory index value , n = z , of the measured data point in the interferogram being corrected . as discussed above d is usefully d max , being that shift d which when applied to the target interferogram using the transform ( 2 ) above will provide a maximum correlation between the reference 40 and the target 42 interferograms .