fig1 shows in plan view the first embodiment of the inventive absorption photometer , in which reference number 1 denotes a light source , e . g ., deuterium ( d 2 ) lamp , 2 is a condensor mirror having a toroidal surface for example , 4 is an incident slit , 5 is a beam splitter , e . g ., semi - transparent mirror , 6 and 8 are planar mirrors , 7 is a short - cut filter for transmitting the light above a certain wavelength , 9 is a concave diffraction grating , 3 is an output slit , 10 is a wavelength bar , 11 is a dial , 12 is a wavelength cam , 13 is a flow cell in which a liquid sample is fed for the continuous measurement , 15 is a diffraction grating , 16 is a measuring detector , 17 is a short - cut filter , 18 is a reference detector , and 19 is a signal processing circuit . a condenser lens 14 may be inserted immediately after the flow cell 13 . a light shield 100 is provided in the exterior of the light path . in the absorption photometer , the light from the light source 1 is condensed by the condensor mirror 2 , and it is entered through the incident slit 4 into the spectrometer . the main light beam is reflected by the planar mirror 8 , and , after being entered at a smaller incident angle and dispersed by the concave grating 9 , the light with a specific wavelength goes through the output slit 3 . part of the light is directed apart from the main light beam by means of the beam splitter 5 in the spectrometer , and , after being incident through the mirror 6 and short - cut filter 7 to the concave grating 9 at a larger incident angle and dispersed by it , the light above the above - mentioned specific wavelength goes through the output slit 3 . for a grating , there is given for the first - order diffraction light a functional relationship between the light wavelength λ , grating incident angle α , diffraction angle β and grating constant d , as follows : sin α + sin β = λ / d . in fig1 the incident angles α 1 and α 2 and output angle β are all positive , and the output light from the output slit 3 includes two kinds of light . since β is common , the light with the greater incident angle will have a longer wavelength . with the incident angles α 1 and α 2 ( α 1 & lt ; α 2 ) being made correspondence to wavelengths λ 2 = 250 nm and λ 2 = 400 nm , respectively , the angles α 1 , α 2 and β are chosen . the light which has passed through the output slit 3 is directed to the flow cell 13 . the flow cell 13 has a flow of liquid , and the concentration of a certain component in the liquid is measured continuously . when the sample is little in quantity , a flow cell with a smaller inner diameter is preferably used . however , it is difficult for a flow cell with a smaller inner diameter to maintain the light path in the cell , and the measuring wall of the cell is prone to cut part of the light beam . when a single light beam including more than one wavelength is conducted through the flow cell to implement monitoring using a wavelength free from absorption , the variation in the amount of light can be corrected even in the aforementioned case . this allows the use of a flow cell of 0 . 4 mm in inner diameter and 1 cm in length . the need of using a cell shorter than 1 cm in reference length with the intention of maintaining the light path is reduced significantly . behind the flow cell 13 , there is disposed a diffraction grating 15 . a lens 14 may be provided so that the condensed light is incident to the grating 15 . in this embodiment , the grating 15 plays two roles . one is to diffract the zero - order light in the same way as mirror reflection , and the other is to disperse the light ( particularly first - order dispersion ) depending on the wavelength . the measuring light coming by way of the mirror 8 undergoes first - order diffraction , and it is received by the measuring detector 16 . the reference light coming by way of the mirror 6 and short - cut filter 7 undergoes zero - order diffraction ( reflection ), and it is fed through the short - cut filter 17 and received by the reference detector 18 . the use of the zero - order light is advantageous in the efficiency of energy , and it also allows a greater angle between the measuring light and reference light . the measuring light has its zero - order light diffracted in the same direction , but it is removed by the shortcut filter 17 . the detector is constituted , for example , by a silicon photocell , or otherwise a photomultiplier , phototube , or the like may be employed for it . a concave grating with a short focal distance may be used for the grating 15 , and in this case the lens 14 is unused . the reference detector may be placed at the position of receiving the first - order dispersion light of the reference light , instead of placing it at the position of receiving the zero - order light . in this case , dispersed lights are received and the shortcut filter 17 may be unused . the reference detector and measuring detector may be integrated to form a pair cell so that the number of component parts is reduced . particularly , when two light spots to be detected are close to each other , disposition of component parts is facilitated . fig2 is pertinent to the detection system of the embodiment shown in fig1 including the grating 15 , measuring detector 16 and reference detector 17 , explaining the reason for the reception of the first - order diffraction light with the measuring detector 16 and the reception of the zero - order light with the reference detector 17 . the energy efficiency (%) is plotted on the ordinate versus the wavelength ( nm ) on the abscissa . the grating 15 is assumed to have its blaze wavelength set to somewhere about 200 nm . the first - order diffraction light has a higher energy efficiency in the ultraviolet range centered by about 200 nm . as the wavelength increases , the energy efficiency of the first - order diffraction light falls . the zero - order light has its energy efficiency rising with the increase in the wavelength . in many cases , the zero - order light has a higher energy efficiency in the visual range . the reference light is set to have a sufficiently longer wavelength ( e . g ., 100 - 300 nm ) relative to the measuring light . in case the zero - order light has a higher energy efficiency than the first - order light at the reference light wavelength , it is advantageous to use the first - order diffraction light for the measuring light and the zero - order light for the reference light . on this account , preferably , the measuring light is set to have a wavelength somewhere about the absorption peak wavelength of the substance under measurement and the reference light is set to have a wavelength longer by 100 - 300 nm than the measuring light where the absorption by the substance under measurement is negligible . fig3 a and 3b show the effect of correction by the light beam monitor . the absorbance output is plotted on the ordinate versus the time on the abscissa . fig3 b is the case without correction for the downstream portion after the flow cell , while fig3 a is the case with correction for the downstream portion after the flow cell . for example , a possible matter is that the liquid flowing in the flow cell has its refractive index varied due to changes in the pressure , temperature and the like caused by changes in the mixing ratio and flowrate , resulting in the variation of the optical axis , and thence in the variation in the amount of light on the detector . if correction is not conducted , a change in the base line as shown in fig3 b will arise in the measurement of absorption . the absorption photometer which also monitors changes inside the flow cell as mentioned above is subjected to virtually equal changes for the measuring light and reference light . the output signal of the measuring light calibrated by the reference light is modified as shown in fig3 a . also in the case of the positional variation of the light source or the shift of optical axis caused by the inhomogeneity of air , which arises often with such a light source using the arc discharge as a d 2 lamp , the foregoing absorption photometer is capable of accurately correcting the variation , since the reference light goes through the flow cell as the measuring light does . in another case where the reference light does not go through the flow cell , but is projected to a detector by way of another slit or the like , it is difficult to reproduce the same condition a the flow cell and to make a fine adjustment for the positional correspondence between the flow cell and the slit on the part of the reference light . fig4 shows the optical system for explaining the second embodiment of this invention . besides the component parts similar to those of fig1 as referred to by similar symbols , the system includes a lens 20 , a beam splitter 21 , a mirror 22 , a first incident slit 23 , a second incident slit 24 , a beam splitter 25 , a band - pass filter 26 , and a short - cut filter 27 . in the absorption photometer of this embodiment , the light from the light source 1 is condensed by the lens 20 . after being split by the beam splitter 21 , two light beams enter the spectrometer by way of the first incident slit 23 and second incident slit 24 . the beams are dispersed by the concave grating 9 , and two kinds of dispersion lights are distributed . among the dispersed lights of the light incident through the first incident slit 23 , the light with a specific wavelength ( λ 1 ) goes through the flow cell 13 by way of the exit slit 3 . the light beam split by the beam splitter 21 is reflected by the mirror 22 , and it irradiates the second incident slit 24 . in case the lens 20 has the chromatic aberration , a longer wavelength light has a longer focal distance . in the optical system of fig4 the fact that the second light beam going through the beam splitter 21 and mirror 22 has a longer light path than that of the first light beam can also be utilized positively for the correction of chromatic aberration . the second light beam is incident to the concave grating 9 at an incident angle different from that of the first light beam entering through the first incident slit , and it supplies the light with a wavelength ( λ 2 ) different from the measuring wavelength λ 1 to the flow cell 13 by way of the exit slit 3 . in the disposition of fig4 λ 2 is larger in value , i . e ., longer wavelength , than λ 1 . the light beams with two distinct wavelengths passing through the flow cell 13 are divided by the dichroic mirror 25 of dielectric multi - layer films so that the light with the shorter wavelength λ 1 is reflected and the light with the longer wavelength λ 2 is transmitted . a band - pass filter or the like may be used in conjunction . the light with wavelength λ 2 which has passed through the dichroic mirror 25 is incident to the reference detector 18 . both detectors 16 and 18 have their output signals fed to a ratio calculator 102 , in which the sample signal s is divided by the reference signal r , and the resulting s / r signal is sent to a logarithmic converter 104 . the logarithmic converter 104 produces the absorbance signal given by log 10 ( io / i )= c log ( s / r ), where io is the incident light intensity and i is the transmitted light intensity . instead of separating lights with two kinds of wavelengths using the dichroic mirror and filter after transmission through the flow cell , they can also be separated using a diffraction grating or prism . fig5 shows the optical system according to the third embodiment of this invention . besides the component parts similar to those in fig1 and 4 as referred to by similar symbols , the system includes a sector mirror 28 and a detector 29 . in this embodiment , the beam splitter 21 used in the second embodiment of fig4 is replaced with the sector mirror 28 , and the light is distributed to the first incident slit 23 and second incident slit 24 on a time slice basis . in this case , the flow cell 13 is followed in its rear by the sole detector 29 , which provides an output signal indicative of the measurement signal and reference signal alternately . these signals can be extracted separately by sampling each signal synchronously with the sector mirror 28 by means of a synchronous detection circuit 108 such as a lock - in amplifier . fig6 shows the optical system according to the fourth embodiment of this invention . in the figure , component parts similar to those in fig1 and 5 are referred to by similar symbols . in this embodiment , the light beam is not split at all before it reaches the flow cell 13 , and the light coming through the incident slit 4 is simply analyzed and fed toward the flow cell 13 by way of the output slit 3 . the concave grating 9 has a blaze wavelength in the long wavelength range above 400 nm so that the second - order diffraction light with wavelength λ 1 in the ultraviolet range and the first - order diffraction light with wavelength λ 2 in the visual range pass together through the flow cell 13 . after passing through the flow cell 13 , the light with each wavelength is extracted . the light reflected upward by a beam splitter 25 such as a semi - transparent mirror is conducted through a combination filter 26 made up of a band - pass filter and color glass filter so that only the light with wavelength λ 1 is selected . another light beam split by the beam splitter is fed through a short - cut filter 27 so that the second - order light is removed . when the presence of high - order lights does not matter , the color glass filter may be omitted . according to the absorption photometers described in the foregoing embodiments , the variation arising from a cut of light as being interrupted partially by the interior wall of the flow cell can be corrected through the monitoring at a downstream portion after the flow cell . accordingly , even if the light source spot moves , the optical axis shifts due to the movement of air or the density of liquid in the flow cell varies , the stable measurement is promised invariably . although liquid chromatograph has been explained as an example , the present invention is not confined to this . the inventive photometer is particularly effective for the optical measurement using a flow cell in which liquid flows . for the light source , a tungsten lamp , metallic halide lamp or the like can be used selectively for each purpose .